[llvm] [AMDGPU] AsmPrinter: Unify arg handling (PR #151672)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> This pretty much forcesus to keep the IR Function around forever and repeats some of the logic from instruction selection.

The full IR always survives the full compilation. It would kind of be better if we had all the utils to query all ABI details directly computed from the IR signature 

https://github.com/llvm/llvm-project/pull/151672