[llvm] [WIP][AMDGPU] Improve the handling of `inreg` arguments (PR #133614)

[Shilei Tian via llvm-commits]

shiltian wrote:

I'm still working on this, but I'd like to get some early feedback. The newly added test case currently crashes in the verifier with the following error:

```
error: Illegal instruction detected: Illegal immediate value for operand.
renamable $vgpr20 = V_WRITELANE_B32 killed $sgpr0, killed $sgpr1, 0
error: Illegal instruction detected: Illegal immediate value for operand.
renamable $vgpr1 = V_WRITELANE_B32 killed $sgpr0, killed $sgpr1, 1
```

Looks like I can't just use an immediate operand for the lane selection. I'll fix that in a later update.

https://github.com/llvm/llvm-project/pull/133614