[llvm] [NFCI][IR] Add optional DataLayout argument to zero and null value related APIs (PR #183208)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Does it make sense that we change all existing uses of `isNullValue` to `isZeroValue` first, and then add back `isNullValue` along with the semantics change of `ConstantPointerNull` only for pointers (as well as vector of pointers)?

I think so (though I would maybe name it isNullPtrValue? Though wouldn't that only be covered by ConstantPointerNull?)

https://github.com/llvm/llvm-project/pull/183208