[llvm] [BasicTTI] Use getTypeLegalizationCost to generalize vector cast cost. (PR #107303)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Please can you look at the CI test failures?
```
Failed Tests (2):
  LLVM :: Analysis/CostModel/AArch64/fp-conversions-odd-vector-types.ll
  LLVM :: Transforms/LoopVectorize/AArch64/sve-inductions-unusual-types.ll
```

https://github.com/llvm/llvm-project/pull/107303