[HLSL] Determine the minimal set of passes required to legalize HLSL for DirectX and SPIR-V when compiling with `-O0`

[Icohedron]

Once the OptimizeNone attribute is not interfering with the application of necessary optimization passes for legalization (#151564) we need to determine the minimal set of optimization passes required to legalize HLSL for DirectX/DXIL and SPIR-V to add to clang's BackendUtil when compiling with -O0.