[llvm] [AMDGPU] Create an AMDGPUIfConverter pass (PR #106415)

[Juan Manuel Martinez Caamaño via llvm-commits]

jmmartinez wrote:

> As this matures, does it makes sense to separate the target-independent portion into an isolated patch? Or at least provide a more generic title so other downstream users don't overlook the target-independent changes.

Yes, I will do that. I wanted to get some quick validation of this work before being dug in.

I think I'm going to start by:
1. moving `EarlyIfConverter` and `EarlyIfPredicator` to the new pass manager.
2. factorizing the boolean argument into the `PredicationStrategy`
3. move the `shouldIfConvert` into the `PredicationStrategy`
4. move the post-dominator tree traversal and the `tryIfConvert` function into `SSAIfConv`.

Only then I can start with the AMDGPU target specific stuff.

Thanks for the feedback ! 

https://github.com/llvm/llvm-project/pull/106415