[llvm] [SystemZ] Eliminate call sequence instructions early. (PR #77812)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

Yes, I think this is really something that should be fixed in the register allocate (looks like a parallel-copy problem).  But for now I think this patch is fine as is.  Feel free to add a TODO to the test case, otherwise this LGTM.

https://github.com/llvm/llvm-project/pull/77812