[llvm] [SystemZ] Avoid unaligned/overlapping accesses with memcpy/memset. (PR #187100)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

> Rebased on top of pre-committed tests.

Thanks!  If performance results are good, this looks reasonable to me as a first step.

https://github.com/llvm/llvm-project/pull/187100