[DirectX] Scalarize `extractelement` and `insertelement` with dynamic indices (#141676)
Fixes #141136
- Implement `visitExtractElementInst` and `visitInsertElementInst` in
`DXILDataScalarizerVisitor` to scalarize `extractelement` and
`insertelement` instructions whose index operand is not a `ConstantInt`
by converting the vector to an array and then loading from the array
- Rename the `replaceVectorWithArray` helper function to
`equivalentArrayTypeFromVector`, relocate the function toward the top of
the file, and remove the unused `Ctx` parameter