cmd/compile: add MOVBEWstore support for GOAMD64>=3

cmd/compile: add MOVBEWstore support for GOAMD64>=3

This CL add MOVBE support for 16-bit version, but MOVBEWload is
excluded because it does not satisfy zero extented.

For #51724

Change-Id: I3fadf20bcbb9b423f6355e6a1e340107e8e621ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/396617
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>