[SystemZ] Allow any I5 in RotateSelect*

For all RotateSelect* instructions, PoP says:

    Bits 0-1 of the I5 field (bits 32-33 of the instruction) are
    ignored.

LLVM, however, completely prohibits using them, e.g.:

    error: invalid operand for instruction
    asm("rxsbg %[r1],%[r2],177,43,228\n"

Lift this unnecessary restriction.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D146185