[MachineVerifier] Analyze MachineMemOperands for mem-to-mem moves.

Since the verifier will give false reports if it incorrectly thinks MI is
loading or storing using an FI, it is necessary to scan memoperands and
find out how the FI is used in the instruction. This should be relatively
rare.

Needed to make CodeGen/SystemZ/spill-01.ll pass, which now runs with this flag.

Reviewed by Quentin Colombet.

llvm-svn: 251620