Let MachineVerifier be aware of mem-to-mem instructions.
A mem-to-mem instruction (that both loads and stores), which store to an
FI, cannot pass the verifier since it thinks it is loading from the FI.
For the mem-to-mem instruction, do a looser check in visitMachineOperand()
and only check liveness at the reg-slot while analyzing a frame index operand.
Needed to make CodeGen/SystemZ/xor-01.ll pass with -verify-machineinstrs,
which now runs with this flag.
Reviewed by Evan Cheng and Quentin Colombet.