cmd/compile: fix buggy AMD64 rewrite from CL 213058

CL 213058's "bonus optimization I noticed while working on this"
turns out to be buggy. It would be correct for CMP, but not TEST.
Fix it to use TEST semantics instead.

This was breaking compilation with the upcoming Spectre mode.

Change-Id: If2d4c3798ed182f35f0244febe74e68c61e4c61b
Reviewed-on: https://go-review.googlesource.com/c/go/+/222853


Reviewed-by: Keith Randall <khr@golang.org>