The per-Value slice of liveness maps is currently one of the largest
sources of allocation in the compiler. On cmd/compile/internal/ssa,
it's 5% of overall allocation, or 75MB in total. Enabling liveness
maps everywhere significantly increased this allocation footprint,
which in turn slowed down the compiler.

Improve this by compacting the liveness maps after every block is
processed. There are typically very few distinct liveness maps, so
compacting the maps after every block, rather than at the end of the
function, can significantly reduce these allocations.

Passes toolstash -cmp.

name        old time/op       new time/op       delta
Template          198ms ± 2%        196ms ± 1%  -1.11%  (p=0.008 n=9+10)
Unicode           100ms ± 1%         99ms ± 1%  -0.94%  (p=0.015 n=8+9)
GoTypes           703ms ± 2%        695ms ± 1%  -1.15%  (p=0.000 n=10+10)
Compiler          3.38s ± 3%        3.33s ± 0%  -1.66%  (p=0.000 n=10+9)
SSA               7.96s ± 1%        7.93s ± 1%    ~ 	(p=0.113 n=9+10)
Flate             134ms ± 1%        132ms ± 1%  -1.30%  (p=0.000 n=8+10)
GoParser          165ms ± 2%        163ms ± 1%  -1.32%  (p=0.013 n=9+10)
Reflect           462ms ± 2%        459ms ± 0%  -0.65%  (p=0.036 n=9+8)
Tar               188ms ± 2%        186ms ± 1%    ~     (p=0.173 n=8+10)
XML               243ms ± 7%        239ms ± 1%    ~     (p=0.684 n=10+10)
[Geo mean]        421ms             416ms       -1.10%

name        old alloc/op      new alloc/op      delta
Template         38.0MB ± 0%       36.5MB ± 0%  -3.98%  (p=0.000 n=10+10)
Unicode          30.3MB ± 0%       29.6MB ± 0%  -2.21% 	(p=0.000 n=10+10)
GoTypes           125MB ± 0%        120MB ± 0%  -4.51% 	(p=0.000 n=10+9)
Compiler          575MB ± 0%        546MB ± 0%  -5.06% 	(p=0.000 n=10+10)
SSA              1.64GB ± 0%       1.55GB ± 0%  -4.97% 	(p=0.000 n=10+10)
Flate            25.9MB ± 0%       25.0MB ± 0%  -3.41% 	(p=0.000 n=10+10)
GoParser         30.7MB ± 0%       29.5MB ± 0%  -3.97% 	(p=0.000 n=10+10)
Reflect          84.1MB ± 0%       81.9MB ± 0%  -2.64% 	(p=0.000 n=10+10)
Tar              37.0MB ± 0%       35.8MB ± 0%  -3.27% 	(p=0.000 n=10+9)
XML              47.2MB ± 0%       45.0MB ± 0%  -4.57% 	(p=0.000 n=10+10)
[Geo mean]       83.2MB            79.9MB       -3.86%

name        old allocs/op     new allocs/op     delta
Template           337k ± 0%         337k ± 0%  -0.06%  (p=0.000 n=10+10)
Unicode            340k ± 0%         340k ± 0%  -0.01% 	(p=0.014 n=10+10)
GoTypes           1.18M ± 0%        1.18M ± 0%  -0.04% 	(p=0.000 n=10+10)
Compiler          4.97M ± 0%        4.97M ± 0%  -0.03% 	(p=0.000 n=10+10)
SSA               12.3M ± 0%        12.3M ± 0%  -0.01% 	(p=0.000 n=10+10)
Flate              226k ± 0%         225k ± 0%  -0.09% 	(p=0.000 n=10+10)
GoParser           283k ± 0%         283k ± 0%  -0.06% 	(p=0.000 n=10+9)
Reflect            972k ± 0%         971k ± 0%  -0.04% 	(p=0.000 n=10+8)
Tar                333k ± 0%         332k ± 0%  -0.05% 	(p=0.000 n=10+9)
XML                395k ± 0%         395k ± 0%  -0.04% 	(p=0.000 n=10+10)
[Geo mean]         764k              764k       -0.04%

Updates #24543.

Change-Id: I6fdc46e4ddb6a8eea95d38242345205eb8397f0b
Reviewed-on: https://go-review.googlesource.com/110177
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>