[llvm] [clang] [SimplifyCFG] Not folding branch in loop header with constant iterations (PR #74268)

[via cfe-commits]

xiangzh1 wrote:

> AMDGPU can not unorll this case:
> 
> https://godbolt.org/z/4Pq3bnzTT
> 
> But the same code in X86 looks can unroll:
> 
> https://godbolt.org/z/zr8aTG1KW
> 
> We may need to continue debug on it.

X86 do very conservative unroll too，its upper bound send to 4 (default is 8), if we not fold the loop branch, it can fully unroll (16)

https://github.com/llvm/llvm-project/pull/74268