[dev.simd] cmd/compile: add masked merging ops and optimizations

[dev.simd] cmd/compile: add masked merging ops and optimizations

This CL generates optimizations for masked variant of AVX512
instructions for patterns:

x.Op(y).Merge(z, mask) => OpMasked(z, x, y mask), where OpMasked is
resultInArg0.

Change-Id: Ife7ccc9ddbf76ae921a085bd6a42b965da9bc179
Reviewed-on: https://go-review.googlesource.com/c/go/+/718160
Reviewed-by: David Chase <drchase@google.com>
TryBot-Bypass: Junyang Shao <shaojunyang@google.com>