[InstCombine] powi(x, y) * powi(x, z) -> powi(x, y + z)

We already have pow(x, y) * pow(x, z) -> pow(x, y + z) transformation, but we are missing same transformation for powi (power is integer).

Requires reassoc.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D109954