[SystemZ] Reimplement the i8/i16 compare-and-swap logic.

Even though the implementation in emitAtomicCmpSwapW() was correct, it made
Valgrind report an error. Instead of using a RISBG on CmpVal, an LL[CH]R can
be made on the OldVal, and the problem is avoided.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D97604