[spirv][math] Fix sign propagation for math.powf conversion

For `x^y`, the result's sign should consider whether `y` is
an integer and whether it's odd or even.

This still does not cover all corner cases regarding `x^y`
but it's an improvement over the current implementation.

Reviewed By: antiagainst, qedawkins

Differential Revision: https://reviews.llvm.org/D150234