cmd/compile/internal/ssa: combine zero stores into larger stores on arm64

cmd/compile/internal/ssa: combine zero stores into larger stores on arm64

This reduces the go tool binary on arm64 by 12k.

go1 results on Amberwing:
name                   old time/op    new time/op    delta
RegexpMatchEasy0_32       249ns ± 0%     249ns ± 0%    ~     (p=0.087 n=10+10)
RegexpMatchEasy0_1K       584ns ± 0%     584ns ± 0%    ~     (all equal)
RegexpMatchEasy1_32       246ns ± 0%     246ns ± 0%    ~     (p=1.000 n=10+10)
RegexpMatchEasy1_1K       806ns ± 0%     806ns ± 0%    ~     (p=0.706 n=10+9)
RegexpMatchMedium_32      314ns ± 0%     314ns ± 0%    ~     (all equal)
RegexpMatchMedium_1K     52.1µs ± 0%    52.1µs ± 0%    ~     (p=0.245 n=10+8)
RegexpMatchHard_32       2.75µs ± 1%    2.75µs ± 1%    ~     (p=0.690 n=10+10)
RegexpMatchHard_1K       78.9µs ± 0%    78.9µs ± 1%    ~     (p=0.295 n=9+9)
FmtFprintfEmpty          58.5ns ± 0%    58.5ns ± 0%    ~     (all equal)
FmtFprintfString          112ns ± 0%     112ns ± 0%    ~     (all equal)
FmtFprintfInt             117ns ± 0%     116ns ± 0%  -0.85%  (p=0.000 n=10+10)
FmtFprintfIntInt          181ns ± 0%     181ns ± 0%    ~     (all equal)
FmtFprintfPrefixedInt     222ns ± 0%     224ns ± 0%  +0.90%  (p=0.000 n=9+10)
FmtFprintfFloat           318ns ± 1%     322ns ± 0%    ~     (p=0.059 n=10+8)
FmtManyArgs               736ns ± 1%     735ns ± 0%    ~     (p=0.206 n=9+9)
Gzip                      437ms ± 0%     436ms ± 0%  -0.25%  (p=0.000 n=10+10)
HTTPClientServer         89.8µs ± 1%    90.2µs ± 2%    ~     (p=0.393 n=10+10)
JSONEncode               20.1ms ± 1%    20.2ms ± 1%    ~     (p=0.065 n=9+10)
JSONDecode               94.2ms ± 1%    93.9ms ± 1%  -0.42%  (p=0.043 n=10+10)
GobDecode                12.7ms ± 1%    12.8ms ± 2%  +0.94%  (p=0.019 n=10+10)
GobEncode                12.1ms ± 0%    12.1ms ± 0%    ~     (p=0.052 n=10+10)
Mandelbrot200            5.06ms ± 0%    5.05ms ± 0%  -0.04%  (p=0.000 n=9+10)
TimeParse                 450ns ± 3%     446ns ± 0%    ~     (p=0.238 n=10+9)
TimeFormat                485ns ± 1%     483ns ± 1%    ~     (p=0.073 n=10+10)
Template                 90.4ms ± 0%    90.7ms ± 0%  +0.29%  (p=0.000 n=8+10)
GoParse                  6.01ms ± 0%    6.03ms ± 0%  +0.35%  (p=0.000 n=10+10)
BinaryTree17              11.7s ± 0%     11.7s ± 0%    ~     (p=0.481 n=10+10)
Revcomp                   669ms ± 0%     669ms ± 0%    ~     (p=0.315 n=10+10)
Fannkuch11                3.40s ± 0%     3.37s ± 0%  -0.92%  (p=0.000 n=10+10)
[Geo mean]               67.9µs         67.9µs       +0.02%

name                   old speed      new speed      delta
RegexpMatchEasy0_32     128MB/s ± 0%   128MB/s ± 0%  -0.08%  (p=0.003 n=8+10)
RegexpMatchEasy0_1K    1.75GB/s ± 0%  1.75GB/s ± 0%    ~     (p=0.642 n=8+10)
RegexpMatchEasy1_32     130MB/s ± 0%   130MB/s ± 0%    ~     (p=0.690 n=10+9)
RegexpMatchEasy1_1K    1.27GB/s ± 0%  1.27GB/s ± 0%    ~     (p=0.661 n=10+9)
RegexpMatchMedium_32   3.18MB/s ± 0%  3.18MB/s ± 0%    ~     (all equal)
RegexpMatchMedium_1K   19.7MB/s ± 0%  19.6MB/s ± 0%    ~     (p=0.190 n=10+9)
RegexpMatchHard_32     11.6MB/s ± 0%  11.6MB/s ± 1%    ~     (p=0.669 n=10+10)
RegexpMatchHard_1K     13.0MB/s ± 0%  13.0MB/s ± 0%    ~     (p=0.718 n=9+9)
Gzip                   44.4MB/s ± 0%  44.5MB/s ± 0%  +0.24%  (p=0.000 n=10+10)
JSONEncode             96.5MB/s ± 1%  96.1MB/s ± 1%    ~     (p=0.065 n=9+10)
JSONDecode             20.6MB/s ± 1%  20.7MB/s ± 1%  +0.42%  (p=0.041 n=10+10)
GobDecode              60.6MB/s ± 1%  60.0MB/s ± 2%  -0.92%  (p=0.016 n=10+10)
GobEncode              63.4MB/s ± 0%  63.6MB/s ± 0%    ~     (p=0.055 n=10+10)
Template               21.5MB/s ± 0%  21.4MB/s ± 0%  -0.30%  (p=0.000 n=9+10)
GoParse                9.64MB/s ± 0%  9.61MB/s ± 0%  -0.36%  (p=0.000 n=10+10)
Revcomp                 380MB/s ± 0%   380MB/s ± 0%    ~     (p=0.323 n=10+10)
[Geo mean]             56.0MB/s       55.9MB/s       -0.07%

Change-Id: Ia732fa57fbcf4767d72382516d9f16705d177736
Reviewed-on: https://go-review.googlesource.com/96435
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>