[clang] [llvm] [SystemZ] Do not extend integer arguments in z/OS XPLINK64 ABI (PR #206833)

[Ulrich Weigand via cfe-commits]

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uweigand wrote:

This also looks wrong.  This would also be (potentially) applied to incoming arguments, which would be incorrect.  If you simply want to ensure that `i32` is passed in the same registers and stack `i64`, you'll have to add `i32` to the applicable rules below, most notably:
```
  CCIfType<[i64], CCAssignToRegAndStack<[R1D, R2D, R3D], 8, 8>>,
```
This might have even been a pre-existing bug in case we ever use non-extended `i32` as argument.

https://github.com/llvm/llvm-project/pull/206833