This extends the liveness analysis to track registers containing live
pointers. We do this by tracking bitmaps for live pointer registers
in parallel with bitmaps for stack variables.

This does not yet do anything with these liveness maps, though they do
appear in the debug output for -live=2.

We'll optimize this in later CLs:

name        old time/op       new time/op       delta
Template          193ms ± 5%        195ms ± 2%    ~     (p=0.050 n=9+9)
Unicode          97.7ms ± 2%       98.4ms ± 2%    ~     (p=0.315 n=9+10)
GoTypes           674ms ± 2%        685ms ± 1%  +1.72%  (p=0.001 n=9+9)
Compiler          3.21s ± 1%        3.28s ± 1%  +2.28%  (p=0.000 n=10+9)
SSA               7.70s ± 1%        7.79s ± 1%  +1.07%  (p=0.015 n=10+10)
Flate             130ms ± 3%        133ms ± 2%  +2.19%  (p=0.003 n=10+10)
GoParser          159ms ± 3%        161ms ± 2%  +1.51%  (p=0.019 n=10+10)
Reflect           444ms ± 1%        450ms ± 1%  +1.43%  (p=0.000 n=9+10)
Tar               181ms ± 2%        183ms ± 2%  +1.45%  (p=0.010 n=10+9)
XML               230ms ± 1%        234ms ± 1%  +1.56%  (p=0.000 n=8+9)
[Geo mean]        405ms             411ms       +1.48%

No effect on binary size because we're not yet emitting the register
maps.

For #24543.

Change-Id: Ieb022f0aea89c0ea9a6f035195bce2f0e67dbae4
Reviewed-on: https://go-review.googlesource.com/109352
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>