[SystemZ] Patchset for expanding memcpy/memset using at most two stores.

* Set MaxStoresPerMemcpy and MaxStoresPerMemset to 2.

* Optimize stores of replicated values in SystemZ::combineSTORE(). This
  handles the now expanded memory operations and as well some other
  pre-existing cases.

* Reject a big displacement in isLegalAddressingMode() for a vector type.

* Return true from shouldConsiderGEPOffsetSplit().

Reviewed By: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D122105