cmd/compile: rewrite to elide Slicemask from len==c>0 slicing

cmd/compile: rewrite to elide Slicemask from len==c>0 slicing

This might have been something that prove could be educated
into figuring out, but this also works, and it also helps
prove downstream.

Adjusted the prove test, because this change moved a message.

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

Change-Id: I5eabe639eff5db9cd9766a6a8666fdb4973829cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/697715
Commit-Queue: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Bypass: David Chase <drchase@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/708858
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>