This is done for sparse sets already, but it was missing for sparse
maps. Only affects deadstore and regalloc, as they're the only ones that
use sparse maps.

name                 old time/op    new time/op    delta
DSEPass-4               247µs ± 0%     216µs ± 0%  -12.75%  (p=0.008 n=5+5)
DSEPassBlock-4         3.05ms ± 1%    2.87ms ± 1%   -6.02%  (p=0.002 n=6+6)
CSEPass-4              2.30ms ± 0%    2.32ms ± 0%   +0.53%  (p=0.026 n=6+6)
CSEPassBlock-4         23.8ms ± 0%    23.8ms ± 0%     ~     (p=0.931 n=6+5)
DeadcodePass-4         51.7µs ± 1%    51.5µs ± 2%     ~     (p=0.429 n=5+6)
DeadcodePassBlock-4     734µs ± 1%     742µs ± 3%     ~     (p=0.394 n=6+6)
MultiPass-4             152µs ± 0%     149µs ± 2%     ~     (p=0.082 n=5+6)
MultiPassBlock-4       2.67ms ± 1%    2.41ms ± 2%   -9.77%  (p=0.008 n=5+5)

name                 old alloc/op   new alloc/op   delta
DSEPass-4              41.2kB ± 0%     0.1kB ± 0%  -99.68%  (p=0.002 n=6+6)
DSEPassBlock-4          560kB ± 0%       4kB ± 0%  -99.34%  (p=0.026 n=5+6)
CSEPass-4               189kB ± 0%     189kB ± 0%     ~     (all equal)
CSEPassBlock-4         3.10MB ± 0%    3.10MB ± 0%     ~     (p=0.444 n=5+5)
DeadcodePass-4         10.5kB ± 0%    10.5kB ± 0%     ~     (all equal)
DeadcodePassBlock-4     164kB ± 0%     164kB ± 0%     ~     (all equal)
MultiPass-4             240kB ± 0%     199kB ± 0%  -17.06%  (p=0.002 n=6+6)
MultiPassBlock-4       3.60MB ± 0%    2.99MB ± 0%  -17.06%  (p=0.002 n=6+6)

name                 old allocs/op  new allocs/op  delta
DSEPass-4                8.00 ± 0%      4.00 ± 0%  -50.00%  (p=0.002 n=6+6)
DSEPassBlock-4            240 ± 0%       120 ± 0%  -50.00%  (p=0.002 n=6+6)
CSEPass-4                9.00 ± 0%      9.00 ± 0%     ~     (all equal)
CSEPassBlock-4          1.35k ± 0%     1.35k ± 0%     ~     (all equal)
DeadcodePass-4           3.00 ± 0%      3.00 ± 0%     ~     (all equal)
DeadcodePassBlock-4      9.00 ± 0%      9.00 ± 0%     ~     (all equal)
MultiPass-4              11.0 ± 0%      10.0 ± 0%   -9.09%  (p=0.002 n=6+6)
MultiPassBlock-4          165 ± 0%       150 ± 0%   -9.09%  (p=0.002 n=6+6)

Change-Id: I43860687c88f33605eb1415f36473c5cfe8fde4a
Reviewed-on: https://go-review.googlesource.com/98449
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>