mundaym [s390x] cmd/compile: add SSA rules for s390x test-under-mask instruct…

…ions

Allows the extraction of test-under-mask instructions. For example:

before      | after
-------------------------
MOVD $1, R0 | TMLL R1, $1
AND  R1, R0 | BEQ  ...
CMP  R0, $0 |
BEQ  ...    |

Reduces the size of the .text section in cmd/go by ~0.05%.