[llvm] [InstCombine] optimize powi(X,Y)/X with Ofast (PR #67236)

[Nikita Popov via llvm-commits]

nikic wrote:

@jcranmer-intel Thanks for the great analysis! I guess that means that this fold should be restricted to constant powi arguments, where we can make sure there is no overflow.

Also, this means that this existing fold is also incorrect, right? https://github.com/llvm/llvm-project/blob/d13abd1c7260a4dc7b660f40cb50543260ae9e21/llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp#L735-L743 The add there may overflow.

https://github.com/llvm/llvm-project/pull/67236