[InstCombine] optimize powi(X,Y) * X with Ofast

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, 4) is replaced by
X2 = X * X; X2 * X2 in the further step.
Similar to D109954, who requires reassoc.

Fixes https://github.com/llvm/llvm-project/issues/69862.