Currently liveness information is kept in a map keyed by *ssa.Value.
This made sense when liveness information was sparse, but now we have
liveness for nearly every ssa.Value. There's a fair amount of memory
and CPU overhead to this map now.

This CL replaces this map with a slice indexed by value ID.

Passes toolstash -cmp.

name        old time/op       new time/op       delta
Template          197ms ± 1%        194ms ± 1%  -1.60%  (p=0.000 n=9+10)
Unicode           100ms ± 2%         99ms ± 1%  -1.31%  (p=0.012 n=8+10)
GoTypes           695ms ± 1%        689ms ± 0%  -0.94%  (p=0.000 n=10+10)
Compiler          3.34s ± 2%        3.29s ± 1%  -1.26%  (p=0.000 n=10+9)
SSA               8.08s ± 0%        8.02s ± 2%  -0.70%  (p=0.034 n=8+10)
Flate             133ms ± 1%        131ms ± 1%  -1.04%  (p=0.006 n=10+9)
GoParser          163ms ± 1%        162ms ± 1%  -0.79%  (p=0.034 n=8+10)
Reflect           459ms ± 1%        454ms ± 0%  -1.06%  (p=0.000 n=10+8)
Tar               186ms ± 1%        185ms ± 1%  -0.87%  (p=0.003 n=9+9)
XML               238ms ± 1%        235ms ± 1%  -1.01%  (p=0.004 n=8+9)
[Geo mean]        418ms             414ms       -1.06%

name        old alloc/op      new alloc/op      delta
Template         36.4MB ± 0%       35.6MB ± 0%  -2.29%  (p=0.000 n=9+10)
Unicode          29.7MB ± 0%       29.5MB ± 0%  -0.68%  (p=0.000 n=10+10)
GoTypes           119MB ± 0%        117MB ± 0%  -2.30%  (p=0.000 n=9+9)
Compiler          546MB ± 0%        532MB ± 0%  -2.47%  (p=0.000 n=10+10)
SSA              1.59GB ± 0%       1.55GB ± 0%  -2.41%  (p=0.000 n=10+10)
Flate            24.9MB ± 0%       24.5MB ± 0%  -1.77%  (p=0.000 n=8+10)
GoParser         29.5MB ± 0%       28.7MB ± 0%  -2.60%  (p=0.000 n=9+10)
Reflect          81.7MB ± 0%       80.5MB ± 0%  -1.49%  (p=0.000 n=10+10)
Tar              35.7MB ± 0%       35.1MB ± 0%  -1.64%  (p=0.000 n=10+10)
XML              45.0MB ± 0%       43.7MB ± 0%  -2.76%  (p=0.000 n=9+10)
[Geo mean]       80.1MB            78.4MB       -2.04%

name        old allocs/op     new allocs/op     delta
Template           336k ± 0%         335k ± 0%  -0.31%  (p=0.000 n=9+10)
Unicode            339k ± 0%         339k ± 0%  -0.05%  (p=0.000 n=10+10)
GoTypes           1.18M ± 0%        1.18M ± 0%  -0.26%  (p=0.000 n=10+10)
Compiler          4.96M ± 0%        4.94M ± 0%  -0.24%  (p=0.000 n=10+10)
SSA               12.6M ± 0%        12.5M ± 0%  -0.30%  (p=0.000 n=10+10)
Flate              224k ± 0%         223k ± 0%  -0.30%  (p=0.000 n=10+10)
GoParser           282k ± 0%         281k ± 0%  -0.32%  (p=0.000 n=10+10)
Reflect            965k ± 0%         963k ± 0%  -0.27%  (p=0.000 n=9+10)
Tar                331k ± 0%         330k ± 0%  -0.27%  (p=0.000 n=10+10)
XML                393k ± 0%         392k ± 0%  -0.26%  (p=0.000 n=10+10)
[Geo mean]         763k              761k       -0.26%

Updates #24543.

Change-Id: I4cfd2461510d3c026a262760bca225dc37482341
Reviewed-on: https://go-review.googlesource.com/110178
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>