If fast-math is set, then LLVM is free to do the following transformation pow(x, y) * pow(x, z) -> pow(x, y + z). This patch adds this transformation and tests for it. See more https://bugs.llvm.org/show_bug.cgi?id=47205

It handles two cases

- When operands of fmul are different instructions

%4 = call reassoc float @llvm.pow.f32(float %0, float %1)

%5 = call reassoc float @llvm.pow.f32(float %0, float %2)

%6 = fmul reassoc float %5, %4

-->

%3 = fadd reassoc float %1, %2

%4 = call reassoc float @llvm.pow.f32(float %0, float %3)

- When operands of fmul are the same instruction

%4 = call reassoc float @llvm.pow.f32(float %0, float %1)

%5 = fmul reassoc float %4, %4

-->

%3 = fadd reassoc float %1, %1

%4 = call reassoc float @llvm.pow.f32(float %0, float %3)