When allocating registers, before kicking out the existing value,
copy it to a spare register if there is one. So later use of this
value can be found in register instead of reload from spill. This
is very helpful for instructions of which the input and/or output
can only be in specific registers, e.g. DIV on x86, MUL/DIV on
MIPS. May also be helpful in general.

For "go build -a cmd/go" on AMD64, reduce "spilled value remains"
by 1% (not including args, which almost certainly remain).

For the code in issue #16061 on AMD64:
MaxRem-12   111µs ± 1%    94µs ± 0%  -15.38%  (p=0.008 n=5+5)

Go1 benchmark on AMD64:
BinaryTree17-12              2.32s ± 2%     2.30s ± 1%    ~     (p=0.421 n=5+5)
Fannkuch11-12                2.52s ± 0%     2.44s ± 0%  -3.44%  (p=0.008 n=5+5)
FmtFprintfEmpty-12          39.9ns ± 3%    39.8ns ± 0%    ~     (p=0.635 n=5+4)
FmtFprintfString-12          114ns ± 1%     113ns ± 1%    ~     (p=0.905 n=5+5)
FmtFprintfInt-12             102ns ± 6%      98ns ± 1%    ~     (p=0.087 n=5+5)
FmtFprintfIntInt-12          146ns ± 5%     147ns ± 1%    ~     (p=0.238 n=5+5)
FmtFprintfPrefixedInt-12     155ns ± 2%     151ns ± 1%  -2.58%  (p=0.008 n=5+5)
FmtFprintfFloat-12           231ns ± 1%     232ns ± 1%    ~     (p=0.286 n=5+5)
FmtManyArgs-12               657ns ± 1%     649ns ± 0%  -1.31%  (p=0.008 n=5+5)
GobDecode-12                6.35ms ± 0%    6.29ms ± 1%    ~     (p=0.056 n=5+5)
GobEncode-12                5.38ms ± 1%    5.45ms ± 1%    ~     (p=0.056 n=5+5)
Gzip-12                      209ms ± 0%     209ms ± 1%    ~     (p=0.690 n=5+5)
Gunzip-12                   31.2ms ± 1%    31.1ms ± 1%    ~     (p=0.548 n=5+5)
HTTPClientServer-12          123µs ± 4%     130µs ± 8%    ~     (p=0.151 n=5+5)
JSONEncode-12               14.0ms ± 1%    14.0ms ± 1%    ~     (p=0.421 n=5+5)
JSONDecode-12               41.2ms ± 1%    41.1ms ± 2%    ~     (p=0.421 n=5+5)
Mandelbrot200-12            3.96ms ± 1%    3.98ms ± 0%    ~     (p=0.421 n=5+5)
GoParse-12                  2.88ms ± 1%    2.88ms ± 1%    ~     (p=0.841 n=5+5)
RegexpMatchEasy0_32-12      68.0ns ± 3%    66.6ns ± 1%  -2.00%  (p=0.024 n=5+5)
RegexpMatchEasy0_1K-12       728ns ± 8%     682ns ± 1%  -6.26%  (p=0.008 n=5+5)
RegexpMatchEasy1_32-12      66.8ns ± 2%    66.0ns ± 1%    ~     (p=0.302 n=5+5)
RegexpMatchEasy1_1K-12       291ns ± 2%     288ns ± 1%    ~     (p=0.111 n=5+5)
RegexpMatchMedium_32-12      103ns ± 2%     100ns ± 0%  -2.53%  (p=0.016 n=5+4)
RegexpMatchMedium_1K-12     31.9µs ± 1%    31.3µs ± 0%  -1.75%  (p=0.008 n=5+5)
RegexpMatchHard_32-12       1.59µs ± 2%    1.59µs ± 1%    ~     (p=0.548 n=5+5)
RegexpMatchHard_1K-12       48.3µs ± 2%    47.7µs ± 1%    ~     (p=0.222 n=5+5)
Revcomp-12                   340ms ± 1%     338ms ± 1%    ~     (p=0.421 n=5+5)
Template-12                 46.3ms ± 1%    46.5ms ± 1%    ~     (p=0.690 n=5+5)
TimeParse-12                 252ns ± 1%     247ns ± 0%  -1.91%  (p=0.000 n=5+4)
TimeFormat-12                277ns ± 1%     267ns ± 0%  -3.82%  (p=0.008 n=5+5)
[Geo mean]                  48.8µs         48.3µs       -0.93%

It has very little effect on binary size and compiler speed.
compilebench:
Template       230ms ±10%      231ms ± 8%    ~             (p=0.546 n=9+9)
Unicode        123ms ± 6%      124ms ± 9%    ~           (p=0.481 n=10+10)
GoTypes        742ms ± 6%      755ms ± 3%    ~           (p=0.123 n=10+10)
Compiler       3.10s ± 3%      3.08s ± 1%    ~           (p=0.631 n=10+10)

Fixes #16061.

Change-Id: Id99cdc7a182ee10a704fa0f04e8e0d0809b2ac56
Reviewed-on: https://go-review.googlesource.com/29732
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>