[dev.simd] cmd/compile, simd: remove mask load and stores

[dev.simd] cmd/compile, simd: remove mask load and stores

We have convert mask to bits already, the API of mask load and stores
are inconsistent with them, also mask load and stores could just be
hidden behind peepholes. So this CL removes them, the next CL will add
the peephole for them.

Change-Id: Ifa7d23fb52bb0efd1785935ead4d703927f16d2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/710915
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>