Add step builtins and step HLSL function to DirectX and SPIR-V backend (#106471)
This PR adds the step intrinsic and an HLSL function that uses it.
The SPIRV backend is also implemented.
Used https://github.com/llvm/llvm-project/pull/102683 as a reference.
Fixes https://github.com/llvm/llvm-project/issues/99157