[llvm] [AMDGPU] Add getCmpSelInstrCost override to re-enable SimplifyCFG speculation for vector types (PR #208043)

[Matt Arsenault via llvm-commits]

================

----------------
arsenm wrote:

```suggestion
  return getScalarizationOverhead(ValVTy, /*Insert=*/ true,
                                  /*Extract=*/ false, CostKind) +
```

https://github.com/llvm/llvm-project/pull/208043