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

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

> 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.

This PR already migrated the legitimate zeroing cases.  After inspecting the code there's only a couple of bogus use of the constructor so I've created https://github.com/llvm/llvm-project/pull/82810 to fix them and then removed the constructor.

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