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

[Nikita Popov via llvm-commits]

nikic wrote:

Depends a bit on what usages you find, but I'd be partial to not having isNullValue() at all and using `isa<ConstantPointerNull>()` instead...

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