[llvm] [LLVM][SelectionDAG] Reduce number of ComputeValueVTs variants. (PR #75614)

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm approved this pull request.

`TypeSize::getZero()` does the same thing as the constructor `TypeSize()`. The reason for adding `TypeSize()` was so that we could use TypeSize in container classes (e.g. a `SmallVector<TypeSize>`), but I guess the initialisation to `0` is not strictly required.

Could you follow this patch up with a patch that removes the zero-initialisation from the `TypeSize()` constructor and replace all existing uses of `TypeSize()` that expect zero-initialisation by the explicit `TypeSize::getZero()` ? That would having another way of expressing a 'zero' TypeSize.

https://github.com/llvm/llvm-project/pull/75614