[clang] [llvm] [AMDGPU][clang][CodeGen][opt] Add late-resolved feature identifying predicates (PR #134016)

[Dmitry Sidorov via llvm-commits]

MrSidims wrote:

> Thank you for the feedback! I might not be getting the question right (case in which I apologise in advance!), but I think that for "vanilla" SPIR-V i.e. not vendor flavoured one, where one strictly has to deal with Extensions / non-core capabilities, we probably would have the following situation:

I was imagining cases like this:
```
if (__builtin_amdgcn_processor_is("some_hw_with_fp16_support) {
/*code using fp16*/
} else {
/*code using fp32*/
}
```
note, that when translated to SPIR-V the SPIR-V generator must insert **Float16** capability (in the beginning of the module). So such tool would need to remove that capability as well.

A side question, is it legal to use the builtin in unstructured control flow, like here: https://godbolt.org/z/qnhKdhfdW ?

https://github.com/llvm/llvm-project/pull/134016