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

Try to transform the powi(X, Y) / X into powi(X, Y-1) with Ofast.
For this case, when the Y is 3, then powi(X, 2) is replaced by X * X in
the further step.

Fixes https://github.com/llvm/llvm-project/pull/67216
Reviewed By: dtcxzyw, nikic, jcranmer-intel