cmd/compile: improve LoweredMove performance on ppc64x

cmd/compile: improve LoweredMove performance on ppc64x

This change improves the performance for LoweredMove on ppc64le
and ppc64.

benchmark                   old ns/op     new ns/op     delta
BenchmarkCopyFat8-16        0.93          0.69          -25.81%
BenchmarkCopyFat12-16       2.61          1.85          -29.12%
BenchmarkCopyFat16-16       9.68          1.89          -80.48%
BenchmarkCopyFat24-16       4.48          1.85          -58.71%
BenchmarkCopyFat32-16       6.12          1.82          -70.26%
BenchmarkCopyFat64-16       21.2          2.70          -87.26%
BenchmarkCopyFat128-16      29.6          3.97          -86.59%
BenchmarkCopyFat256-16      52.6          13.4          -74.52%
BenchmarkCopyFat512-16      97.1          18.7          -80.74%
BenchmarkCopyFat1024-16     186           35.3          -81.02%

BenchmarkAssertE2TLarge-16      14.2          5.06          -64.37%

Fixes #19785

Change-Id: I7d5e0052712b75811c02c7d86c5112e5649ad782
Reviewed-on: https://go-review.googlesource.com/38950
Reviewed-by: Keith Randall <khr@golang.org>