[SPIRV][DirectX][HLSL] Move `step` implementation to `hlsl_intrinsics.h`

[farzonl]

1. Remove Intrinsic::dx_step and Intrinsic::spv_step
2. Remove BI__builtin_hlsl_step
3. Remove BI__builtin_hlsl_step Sema checks from clang/lib/Sema/SemaHLSL.cpp
4. Remove getStepIntrinsic from clang/lib/CodeGen/CGHLSLRuntime.h
5. Remove emitting intrinsic from clang/lib/CodeGen/CGBuiltin.cpp
6. Remove expandStepIntrinsic in llvm/lib/Target/DirectX/DXILIntrinsicExpansion.cpp
7. Implement step algorithm in hlsl_intrinsics.h via tablegen
8. add a pattern match for step as an InstCombine in llvm/lib/Target/SPIRV/SPIRVPreLegalizerCombiner.cpp to be able to still emit the lerp GL extension.

[farzonl]

For any future implementer I would like the pattern matching to be a separate pr from the movement to the code from the backends to the HLSL header.