[DirectX] The `dx.dot2/3/4` and `dx.dot2add` intrinsics leave vectors around post-scalarization

[bogner]

The dx.dot2, dot3, dot4, and dot2add intrinsics are defined as taking vector arguments and have special handling to scalarize those arguments in DXILOpLowering. This causes issues, as the vectors don't get entirely cleaned up before we generate DXIL.

From https://github.com/llvm-beanz/offload-test-suite/pull/61:

# .---command stdout------------
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%17 = insertelement <3 x float> undef, float %16, i32 0' in block '#0' of function 'main'.
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%18 = insertelement <3 x float> %17, float %15, i32 1' in block '#0' of function 'main'.
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%19 = insertelement <3 x float> %18, float %14, i32 2' in block '#0' of function 'main'.
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%20 = extractelement <3 x float> %19, i32 0' in block '#0' of function 'main'.
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%21 = extractelement <3 x float> %19, i32 1' in block '#0' of function 'main'.
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%22 = extractelement <3 x float> %19, i32 2' in block '#0' of function 'main'.
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%23 = extractelement <3 x float> %19, i32 0' in block '#0' of function 'main'.
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%24 = extractelement <3 x float> %19, i32 1' in block '#0' of function 'main'.
# | Function: main: error: Instructions must be of an allowed type.
# | note: at '%25 = extractelement <3 x float> %19, i32 2' in block '#0' of function 'main'.
# | Validation failed.

It would be simpler to make these intrinsics match their DXIL ops more closely and simply take scalar arguments. This will allow the scalarizer to do its job earlier in the pipeline.

Alternatively, we could improve the vectorArgExpansion logic in DXILOpLowering to clean up any remaining vector artifacts, but this seems like it would be redundant with the logic in the scalarizer.

[farzonl]

Matching the dot2/3/4 intrinsics to the dxil op would mean doing custom scalarization in IntrinsicExpansion pass. I had a change that did that and I thought the team came to the conclusion we didn't want to do custom scalarizations like that.

Also for the dot2add case that would be interesting because it forces the scalarization into clang codegen.