[InstCombine] Optimize powi(X, Y)/ (X * Z) with Ofast

foldFDivPowDivisor can address A / powi(x, y) to A * powi(x, -y),
while for small const value y, for example y=2, the instcombine will
transform powi(x, 2) to fmul x, x, so it is not optimal for A / powi(x, 2).

Fix https://github.com/llvm/llvm-project/issues/77171