cmd/internal/obj: rename MOVBE{LL,QQ,WW} to just MOVBE{L,Q,W}

The double suffix doesn't seem to serve any purpose, and we can keep
the old spelling as a backwards compatible alias in cmd/asm.

Change-Id: I3f01fc7249fb093ac1b25bd75c1cb9f39b8f62a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/354700


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>