s390x: Enable 1- and 2-byte operands for v-test

s390x: Enable 1- and 2-byte operands for v-test

The v-test operation tests its operand against zero and sets the condition
code accordingly.  So far the operation was only supported for 4- and
8-byte operands.

Lift this restriction and enable 1- and 2-byte operands for v-test, using
the z/Architecture "test under mask" instructions TM, TMY, and TMLL.
Exploit this in the instruction selector, getting rid of the conversion to
a 4-byte operand.  This slightly reduces the generated code on s390x.