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

[via cfe-commits]

bcl5980 wrote:

https://godbolt.org/z/cMeE61bhf
Loop unroll with -unroll-runtime can partial unroll the case.
@nikic It looks if we don't avoid the transform, it will become a runtime unroll. 
@xiangzh1 's solution looks fine to me if we do not involve loopInfo in simplifycfg.

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