The assembler backend fixes too-far conditional branches, but only
for BEQ and like. Add a case for CBZ and like.

Fixes #17925.

Change-Id: Ie516e6c5ca165b582367283a0110f7081e00c214
Reviewed-on: https://go-review.googlesource.com/33304
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>