[llvm] [SystemZ] Add shrink-wrapping support for ELF prologue/epilogue (PR #225240)

[Ulrich Weigand via llvm-commits]

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uweigand wrote:

This doesn't look right.   R11 is still used (and therefore live at block entry) in many blocks, just not *all* of them.  On the other hand, I don't see other targets handling this at all, no matter whether shrink wrapping is enabled or not.

Maybe this whole block is actually not needed at all, because R11 is considered a "reserved" register in functions that require a frame pointer.  Not sure if live tracking is even done at all for reserved registers ...

What happens if we just completely remove this loop?  (Best to also check with `-verify-machineinstrs`.)

https://github.com/llvm/llvm-project/pull/225240