[llvm] [DataLayout] Add isNullPointerAllZeroes (PR #165314)

[Shilei Tian via llvm-commits]

shiltian wrote:

I'm not sure if this one needs to be landed, since it will be superseded by https://github.com/llvm/llvm-project/pull/166667 anyway, and I'm actively working on it.

https://github.com/llvm/llvm-project/pull/165314