[llvm] [WIP][AMDGPU] combine uniform AMDGPU lane Intrinsics (PR #116953)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

I am concerned this does not have enough context to be definitively correct after later transformations. Consider a case like this:


```
  if (ballot_all(x)) {
    uniform_x = readfirstlane x
    speculatable_use(uniform_x)
    ...
  }
```
This transform will then produce:
```
  if (ballot_all(x)) {
    speculatable_use(x)
     ...
  }
```
Which may then later be incorrectly hoisted:


```
  speculatable_use(x) // x is not actually uniform anymore
  if (ballot_all(x)) { ... }
```

There needs to be something tying the uniformity to the use position

https://github.com/llvm/llvm-project/pull/116953