[DirectX] Implement lowering of Sample and SampleLevel (#201477)
Fixes https://github.com/llvm/llvm-project/issues/192547 and
https://github.com/llvm/llvm-project/issues/192550
The Sample and SampleLevel implementations are combined in this single
PR because the code is extremely similar and the changes are small
thanks to the refactoring of prior implementations of sample lowerings.
Assisted-by: Claude Opus 4.6