[SystemZ] Improve foldMemoryOperandImpl().

Swap the compare operands if LHS is spilled while updating the CCMask:s of
the CC users. This is relatively straight forward since the live-in lists for
the CC register can be assumed to be correct during register allocation
(thanks to 659efa21).

Also fold a spilled operand of an LOCR/SELR into an LOC(G).

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D67437