cmd/compile: using new calls, optimize runtime.memequal(x,constant,1)

Proof of concept; also an actual optimization that fires 180 times
in the Go source base.

Change-Id: I5cb87474be764264cde6e4cbcb471ef109306f08
Reviewed-on: https://go-review.googlesource.com/c/go/+/248404


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>