[llvm] [VPlan] Get Addr computation cost with scalar type if it is uniform for gather/scatter. (PR #150371)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM.

Btw, I think this makes more sense if we change the `Type` argument to `getAddressComputationCost` to the type of the pointer, not the value. But we don't do this consistently today.

`LoopVectorizationCostModel::getMemInstScalarizationCost` passes in the pointer type:

```c++
  // Get the cost of the scalar memory instruction and address computation.
  InstructionCost Cost =
      VF.getFixedValue() * TTI.getAddressComputationCost(PtrTy, SE, PtrSCEV);
```

X86 calls it PtrTy:

```c++
InstructionCost getAddressComputationCost(Type *PtrTy, ScalarEvolution *SE,
```

But VectorCombine passes in the element type:

```c++
    ScalarizedCost += TTI.getAddressComputationCost(VecTy->getElementType());
```

As does `VPWidenMemoryRecipe::computeCost`. 

I think we should fix this in a separate PR.

https://github.com/llvm/llvm-project/pull/150371