[llvm] [AArch64][CostModel] Model sve costs for ctpop (PR #192428)

[Paul Walker via llvm-commits]

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

----------------
paulwalker-arm wrote:

```suggestion
    if (ST->isSVEorStreamingSVEAvailable() && MTy.isFixedLengthVector())
      MTy = MVT::getScalableVectorVT(
          MTy.getVectorElementType(), 128 / MTy.getScalarSizeInBits());
```
Now that we have the correct type we should be able to use the existing lookup code? which will also account for the potential cost of promotion.

https://github.com/llvm/llvm-project/pull/192428