[HLSL] add pow library function

This change exposes the pow library function for HLSL, only available for floating point types.
The pow function is supported for all scalar, vector, and matrix types that contain floating point types.

The full documentation of the HLSL pow function is available here:
https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-pow

Reviewed By: bogner

Differential Revision: https://reviews.llvm.org/D156178