cmd/{asm,compile}: replace TESTB op with CMPWconst on s390x

cmd/{asm,compile}: replace TESTB op with CMPWconst on s390x

TESTB was implemented as AND $0xff, Rx, REGTMP. Unfortunately there
is no 3-operand AND-with-immediate instruction and so it was emulated
by the assembler using two instructions.

This CL uses CMPW instead of AND and also optimizes CMPW to use
the chi instruction where possible.

Overall this CL reduces the size of the .text section of the
bin/go binary by ~2%.

Change-Id: Ic335c29fc1129378fcbb1265bfb10f5b744a0f3f
Reviewed-on: https://go-review.googlesource.com/30690
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>