[llvm] [InstCombine] optimize powi(X,Y) * X with Ofast (PR #69998)

[via llvm-commits]

vfdff wrote:

hi @arsenm, @nikic : I have another idea, not sure if it's appropriate?
-  According to above discussion, many other optimizations have similar problems due to the need to recursively determine the operand `fmt attributes` involved. But in fact, the problem has not been exposed for so long. Is it because these `fmt attributes` are set in units of at least one function, that is, IR operands in the same function should be consistent? That's why this problem has not been exposed for so long?
- If so, then we can just keep the same as we are now.


https://github.com/llvm/llvm-project/pull/69998