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

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

This adds to the change in the earlier dev.simd CL.
Generated by arch/internal/simdgen CL 689276 .

Also includes one test for "it at least works once".

Change-Id: I44a268cfc3bea06c5522ac2cfa04fe13a833e1dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/689335
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>