[HLSL] Implement Texture2D::Gather and Texture2D::GatherCmp (#183323)
Add the Gather functions for Texture2D. Variations for all components
are added (Red, Blue, Greed, Alpha). If targeting Vulkan then the
GatherCmp* function for a component other than 0 will result in an
error, as that will lead to invalid SPIR-V.
Part of https://github.com/llvm/llvm-project/issues/175630.
Assisted by: Gemini