cmp/compile: rewrite CMP $0 with TEST

The CMP* family of instructions are longer than their TEST counterparts by one byte.

After this change, my go tool has 13 cmp.*$0x0 instructions, compared to 5612 before.

Change-Id: Ieb87d65657917e494c0e4b711a7ba2918ae27610
Reviewed-on: https://go-review.googlesource.com/21255


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>