Debuggers use DWARF information to find local variables on the
stack and in registers. Prior to this CL, the DWARF information for
functions claimed that all variables were on the stack at all times.
That's incorrect when optimizations are enabled, and results in
debuggers showing data that is out of date or complete gibberish.

After this CL, the compiler is capable of representing variable
locations more accurately, and attempts to do so. Due to limitations of
the SSA backend, it's not possible to be completely correct.

There are a number of problems in the current design. One of the easier
to understand is that variable names currently must be attached to an
SSA value, but not all assignments in the source code actually result
in machine code. For example:

  type myint int
  var a int
  b := myint(int)
and
  b := (*uint64)(unsafe.Pointer(a))

don't generate machine code because the underlying representation is the
same, so the correct value of b will not be set when the user would
expect.

Generating the more precise debug information is behind a flag,
dwarflocationlists. Because of the issues described above, setting the
flag may not make the debugging experience much better, and may actually
make it worse in cases where the variable actually is on the stack and
the more complicated analysis doesn't realize it.

A number of changes are included:
- Add a new pseudo-instruction, RegKill, which indicates that the value
in the register has been clobbered.
- Adjust regalloc to emit RegKills in the right places. Significantly,
this means that phis are mixed with StoreReg and RegKills after
regalloc.
- Track variable decomposition in ssa.LocalSlots.
- After the SSA backend is done, analyze the result and build location
lists for each LocalSlot.
- After assembly is done, update the location lists with the assembled
PC offsets, recompose variables, and build DWARF location lists. Emit the
list as a new linker symbol, one per function.
- In the linker, aggregate the location lists into a .debug_loc section.

TODO:
- currently disabled for non-X86/AMD64 because there are no data tables.

go build -toolexec 'toolstash -cmp' -a std succeeds.

With -dwarflocationlists false:
before: f02812195637909ff675782c0b46836a8ff01976
after:  06f61e8112a42ac34fb80e0c818b3cdb84a5e7ec
benchstat -geomean  /tmp/220352263 /tmp/621364410
completed   15 of   15, estimated time remaining 0s (eta 3:52PM)
name        old time/op       new time/op       delta
Template          199ms ± 3%        198ms ± 2%     ~     (p=0.400 n=15+14)
Unicode          96.6ms ± 5%       96.4ms ± 5%     ~     (p=0.838 n=15+15)
GoTypes           653ms ± 2%        647ms ± 2%     ~     (p=0.102 n=15+14)
Flate             133ms ± 6%        129ms ± 3%   -2.62%  (p=0.041 n=15+15)
GoParser          164ms ± 5%        159ms ± 3%   -3.05%  (p=0.000 n=15+15)
Reflect           428ms ± 4%        422ms ± 3%     ~     (p=0.156 n=15+13)
Tar               123ms ±10%        124ms ± 8%     ~     (p=0.461 n=15+15)
XML               228ms ± 3%        224ms ± 3%   -1.57%  (p=0.045 n=15+15)
[Geo mean]        206ms             377ms       +82.86%

name        old user-time/op  new user-time/op  delta
Template          292ms ±10%        301ms ±12%     ~     (p=0.189 n=15+15)
Unicode           166ms ±37%        158ms ±14%     ~     (p=0.418 n=15+14)
GoTypes           962ms ± 6%        963ms ± 7%     ~     (p=0.976 n=15+15)
Flate             207ms ±19%        200ms ±14%     ~     (p=0.345 n=14+15)
GoParser          246ms ±22%        240ms ±15%     ~     (p=0.587 n=15+15)
Reflect           611ms ±13%        587ms ±14%     ~     (p=0.085 n=15+13)
Tar               211ms ±12%        217ms ±14%     ~     (p=0.355 n=14+15)
XML               335ms ±15%        320ms ±18%     ~     (p=0.169 n=15+15)
[Geo mean]        317ms             583ms       +83.72%

name        old alloc/op      new alloc/op      delta
Template         40.2MB ± 0%       40.2MB ± 0%   -0.15%  (p=0.000 n=14+15)
Unicode          29.2MB ± 0%       29.3MB ± 0%     ~     (p=0.624 n=15+15)
GoTypes           114MB ± 0%        114MB ± 0%   -0.15%  (p=0.000 n=15+14)
Flate            25.7MB ± 0%       25.6MB ± 0%   -0.18%  (p=0.000 n=13+15)
GoParser         32.2MB ± 0%       32.2MB ± 0%   -0.14%  (p=0.003 n=15+15)
Reflect          77.8MB ± 0%       77.9MB ± 0%     ~     (p=0.061 n=15+15)
Tar              27.1MB ± 0%       27.0MB ± 0%   -0.11%  (p=0.029 n=15+15)
XML              42.7MB ± 0%       42.5MB ± 0%   -0.29%  (p=0.000 n=15+15)
[Geo mean]       42.1MB            75.0MB       +78.05%

name        old allocs/op     new allocs/op     delta
Template           402k ± 1%         398k ± 0%   -0.91%  (p=0.000 n=15+15)
Unicode            344k ± 1%         344k ± 0%     ~     (p=0.715 n=15+14)
GoTypes           1.18M ± 0%        1.17M ± 0%   -0.91%  (p=0.000 n=15+14)
Flate              243k ± 0%         240k ± 1%   -1.05%  (p=0.000 n=13+15)
GoParser           327k ± 1%         324k ± 1%   -0.96%  (p=0.000 n=15+15)
Reflect            984k ± 1%         982k ± 0%     ~     (p=0.050 n=15+15)
Tar                261k ± 1%         259k ± 1%   -0.77%  (p=0.000 n=15+15)
XML                411k ± 0%         404k ± 1%   -1.55%  (p=0.000 n=15+15)
[Geo mean]         439k              755k       +72.01%

name        old text-bytes    new text-bytes    delta
HelloSize         694kB ± 0%        694kB ± 0%   -0.00%  (p=0.000 n=15+15)

name        old data-bytes    new data-bytes    delta
HelloSize        5.55kB ± 0%       5.55kB ± 0%     ~     (all equal)

name        old bss-bytes     new bss-bytes     delta
HelloSize         133kB ± 0%        133kB ± 0%     ~     (all equal)

name        old exe-bytes     new exe-bytes     delta
HelloSize        1.04MB ± 0%       1.04MB ± 0%     ~     (all equal)

Change-Id: I991fc553ef175db46bb23b2128317bbd48de70d8
Reviewed-on: https://go-review.googlesource.com/41770Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>