[SystemZ] Improve codegen for memset.

Memset with a constant length was implemented with a single store followed by
a series of MVC:s. This patch changes this so that one store of the byte is
emitted for each MVC, which avoids data dependencies between the MVCs. An
MVI/STC + MVC(len-1) is done for each block.

In addition, memset with a variable length is now also handled without a
libcall. Since the byte is first stored and then MVC is used from that
address, a length of two must now be subtracted instead of one for the loop
and EXRL. This requires an extra check for the one-byte case, which is
handled in a special block with just a single MVI/STC (like GCC).

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D112004