s390x: Introduce and exploit new ALU operator S390_ALU_ILIH

s390x: Introduce and exploit new ALU operator S390_ALU_ILIH

The handlers of Iop_8HLto16, Iop16HLto32, and Iop_32HLto64 in
s390_isel_int_wrk() yield a sequence of "shift", "and", and "or" ALU
operations, the second of which modifies a register returned from a call
to s390_isel_int_expr().  While this approach does not lead to wrong code
generation (because only the register's upper bits are changed which are
not relevant to the IR type), it violates the general "no-modify" rule.

Replace this sequence of ALU operations by a single ALU operation
S390_ALU_ILIH that inserts the low half of its second operand into the
high half of its first operand.  Use the z/Architecture instruction
RISBG ("rotate then insert selected bits") for implementating it.