[HLSL->SPIR-V] Compile SV_StartVertexLocation as BaseVertex and SV_StartInstanceLocation as BaseInstance.

[WangHoi]

Any plan on implement Shader Model 6.8 Extended Command Information ?

See also:
microsoft/DirectXShaderCompiler#6671

[arcady-lunarg]

There are no current plans to implement Shader Model 6 and in general there is no active development effort on the HLSL frontend. However, patches are still welcome and given these HLSL semantics map pretty directly onto SPIR-V decorations it shouldn't be too hard to add.