[dev.simd] cmd/compile: changes for AVX2 SIMD masked load/store

[dev.simd] cmd/compile: changes for AVX2 SIMD masked load/store

This is "glue" changes and hand work for the AVX2
masked loads/stores.  Does not include generated
function/method declarations or intrinsic registration.

Change-Id: Ic95f90b117d0c471f174407ce3f729f1f517b23c
Reviewed-on: https://go-review.googlesource.com/c/go/+/689295
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>