Bias {Eq,Neq}{8,16} to prefer the extension likely to match
their operand's load (if loaded), and elide sign and zero
extending MOV{B,W}, MOV{B,W}Z when their operands are already
appropriately extended.

Change-Id: Ic01b9cab55e170f68fc2369688b50ce78a818608
Reviewed-on: https://go-review.googlesource.com/29236
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>