cmd/compile: improve slicemask removal

cmd/compile: improve slicemask removal

this will be subsumed by pending changes in local slice
representation, however this was easy and works well.

Cherry-picked from the dev.simd branch. This CL is not
necessarily SIMD specific. Apply early to reduce risk.

Change-Id: I5b6eb10d257f04f906be7a8a6f2b6833992a39e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/704876
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/708866
Reviewed-by: David Chase <drchase@google.com>