[llvm] [AArch64][SVE] - Fix crash Possible incorrect use of EVT::getVectorNumElements(). (PR #218733)

[Simon Pilgrim via llvm-commits]

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RKSimon wrote:

This is all you should need:
```
        ElementCount ScaledEC = VT.getVectorElementCount() * Scale;
        MVT NarrowSVT = MVT::getIntegerVT(NarrowBits);
        EVT NarrowVT = EVT::getVectorVT(*DAG.getContext(), NarrowSVT, ScaledEC);
```

https://github.com/llvm/llvm-project/pull/218733