cmd/compile: use shorter versions of zero-extend ops

cmd/compile: use shorter versions of zero-extend ops

Only need to zero-extend to 32 bits and we get the top
32 bits zeroed for free.

Only the WQ change actually generates different code.
The assembler did this optimization for us in the other two cases.
But we might as well do it during SSA so -S output more closely
matches the actual generated instructions.

Change-Id: I3e4ac50dc4da124014d4e31c86e9fc539d94f7fd
Reviewed-on: https://go-review.googlesource.com/23711
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>