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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I agree it would solve the current issue fairly well to add a helper function in MachineBasicBlock that would do this. But on the other hand, I wonder if it's only x86 primarily that needs the CALLSEQ pseudos around to do SP adjustments around each call. If that's the case, maybe the default should actually be to compute the MaxCallFrameSize during instruction selection? One step in that direction then, would be to eliminate them during finalize-isel for SystemZ.
> 

Is it only x86? That would be useful to know

> So this is not only to fix the issue, but also a nice cleanup of the SystemZ backend, I think. Does this make sense?

I think so, but it would help to better understand exactly if/why this is the way it is only for x86

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