cmd/compile/internal/ssa: optimize memory moving on arm64

cmd/compile/internal/ssa: optimize memory moving on arm64

This CL optimizes memory moving with LDP and STP on arm64.

Benchmarks:
name              old time/op  new time/op  delta
ClearFat7-160     1.08ns ± 0%  0.95ns ± 0%  -11.41%  (p=0.008 n=5+5)
ClearFat8-160     0.84ns ± 0%  0.84ns ± 0%   -0.95%  (p=0.008 n=5+5)
ClearFat11-160    1.08ns ± 0%  0.95ns ± 0%  -11.46%  (p=0.008 n=5+5)
ClearFat12-160    0.95ns ± 0%  0.95ns ± 0%     ~     (p=0.063 n=4+5)
ClearFat13-160    1.08ns ± 0%  0.95ns ± 0%  -11.45%  (p=0.008 n=5+5)
ClearFat14-160    1.08ns ± 0%  0.95ns ± 0%  -11.47%  (p=0.008 n=5+5)
ClearFat15-160    1.24ns ± 0%  0.95ns ± 0%  -22.98%  (p=0.029 n=4+4)
ClearFat16-160    0.84ns ± 0%  0.83ns ± 0%   -0.11%  (p=0.008 n=5+5)
ClearFat24-160    2.15ns ± 0%  2.15ns ± 0%     ~     (all equal)
ClearFat32-160    2.86ns ± 0%  2.86ns ± 0%     ~     (p=0.333 n=5+4)
ClearFat40-160    2.15ns ± 0%  2.15ns ± 0%     ~     (all equal)
ClearFat48-160    3.32ns ± 1%  3.31ns ± 1%     ~     (p=0.690 n=5+5)
ClearFat56-160    2.15ns ± 0%  2.15ns ± 0%     ~     (all equal)
ClearFat64-160    3.25ns ± 1%  3.26ns ± 1%     ~     (p=0.841 n=5+5)
ClearFat72-160    2.22ns ± 0%  2.22ns ± 0%     ~     (p=0.444 n=5+5)
ClearFat128-160   4.03ns ± 0%  4.04ns ± 0%   +0.32%  (p=0.008 n=5+5)
ClearFat256-160   6.44ns ± 0%  6.44ns ± 0%   +0.08%  (p=0.016 n=4+5)
ClearFat512-160   12.2ns ± 0%  12.2ns ± 0%   +0.13%  (p=0.008 n=5+5)
ClearFat1024-160  24.3ns ± 0%  24.3ns ± 0%     ~     (p=0.167 n=5+5)
ClearFat1032-160  24.5ns ± 0%  24.5ns ± 0%     ~     (p=0.238 n=4+5)
ClearFat1040-160  29.2ns ± 0%  29.3ns ± 0%   +0.34%  (p=0.008 n=5+5)
CopyFat7-160      1.43ns ± 0%  1.07ns ± 0%  -24.97%  (p=0.008 n=5+5)
CopyFat8-160      0.89ns ± 0%  0.89ns ± 0%     ~     (p=0.238 n=5+5)
CopyFat11-160     1.43ns ± 0%  1.07ns ± 0%  -24.97%  (p=0.008 n=5+5)
CopyFat12-160     1.07ns ± 0%  1.07ns ± 0%     ~     (p=0.238 n=5+4)
CopyFat13-160     1.43ns ± 0%  1.07ns ± 0%     ~     (p=0.079 n=4+5)
CopyFat14-160     1.43ns ± 0%  1.07ns ± 0%  -24.95%  (p=0.008 n=5+5)
CopyFat15-160     1.79ns ± 0%  1.07ns ± 0%     ~     (p=0.079 n=4+5)
CopyFat16-160     1.07ns ± 0%  1.07ns ± 0%     ~     (p=0.444 n=5+5)
CopyFat24-160     1.84ns ± 2%  1.67ns ± 0%   -9.28%  (p=0.008 n=5+5)
CopyFat32-160     3.22ns ± 0%  2.92ns ± 0%   -9.40%  (p=0.008 n=5+5)
CopyFat64-160     3.64ns ± 0%  3.57ns ± 0%   -1.96%  (p=0.008 n=5+5)
CopyFat72-160     3.56ns ± 0%  3.11ns ± 0%  -12.89%  (p=0.008 n=5+5)
CopyFat128-160    5.06ns ± 0%  5.06ns ± 0%   +0.04%  (p=0.048 n=5+5)
CopyFat256-160    9.13ns ± 0%  9.13ns ± 0%     ~     (p=0.659 n=5+5)
CopyFat512-160    17.4ns ± 0%  17.4ns ± 0%     ~     (p=0.167 n=5+5)
CopyFat520-160    17.2ns ± 0%  17.3ns ± 0%   +0.37%  (p=0.008 n=5+5)
CopyFat1024-160   34.1ns ± 0%  34.0ns ± 0%     ~     (p=0.127 n=5+5)
CopyFat1032-160   80.9ns ± 0%  34.2ns ± 0%  -57.74%  (p=0.008 n=5+5)
CopyFat1040-160   94.4ns ± 0%  41.7ns ± 0%  -55.78%  (p=0.016 n=5+4)

Change-Id: I14186f9f82b0ecf8b6c02191dc5da566b9a21e6c
Reviewed-on: https://go-review.googlesource.com/c/go/+/421654
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Eric Fang <eric.fang@arm.com>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>