cmd/compile: shrink liveness maps

The GC maps don't care about trailing non-pointers in args.
Work harder to eliminate them.

This should provide a slight speedup to everything that reads these
maps, mainly GC and stack copying.

The non-ptr-y runtime benchmarks happen to go from having a non-empty
args map to an empty args map, so they have a significant speedup.

name                old time/op  new time/op  delta
StackCopyPtr-8      80.2ms ± 4%  79.7ms ± 2%  -0.63%  (p=0.001 n=94+91)
StackCopy-8         63.3ms ± 3%  59.2ms ± 3%  -6.45%  (p=0.000 n=98+97)
StackCopyNoCache-8   107ms ± 3%    98ms ± 3%  -8.00%  (p=0.000 n=95+88)

It also shrinks object files a tiny bit:

name        old object-bytes  new object-bytes  delta
Template          476kB ± 0%        476kB ± 0%  -0.03%  (p=0.008 n=5+5)
Unicode           218kB ± 0%        218kB ± 0%  -0.09%  (p=0.008 n=5+5)
GoTypes          1.58MB ± 0%       1.58MB ± 0%  -0.03%  (p=0.008 n=5+5)
Compiler         6.25MB ± 0%       6.24MB ± 0%  -0.06%  (p=0.008 n=5+5)
SSA              15.9MB ± 0%       15.9MB ± 0%  -0.06%  (p=0.008 n=5+5)
Flate             304kB ± 0%        303kB ± 0%  -0.29%  (p=0.008 n=5+5)
GoParser          370kB ± 0%        370kB ± 0%  +0.02%  (p=0.008 n=5+5)
Reflect          1.27MB ± 0%       1.27MB ± 0%  -0.07%  (p=0.008 n=5+5)
Tar               421kB ± 0%        421kB ± 0%  -0.05%  (p=0.008 n=5+5)
XML               518kB ± 0%        517kB ± 0%  -0.06%  (p=0.008 n=5+5)
[Geo mean]        934kB             933kB       -0.07%

Note that some object files do grow;
this can happen because some maps that were
duplicates of each others must be stored separately.

Change-Id: Ie076891bd8e9d269ff2ff5435d5d25c721e0e31d
Reviewed-on: https://go-review.googlesource.com/104175


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>