[llvm] [SystemZ] Improve handling of memmoves. (PR #196285)

[Jonas Paulsson via llvm-commits]

JonPsson1 wrote:

Patch rebased on top of #206005, which was committed to main and adds isMemmove() and others to MemOp, so that this patch no longer has any common-code changes.

Across SPEC, some ~330 less libcalls, ~350 more VL and VST instructions (each), and only 11 more MVCRLs, meaning that the pseudo expansion seems to be relatively rare.
 

https://github.com/llvm/llvm-project/pull/196285