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

[Robert Imschweiler via llvm-commits]

ro-i wrote:

> The null pointer (as in ptr null) is always all-zeroes, no matter the address space.

Isn't that what's being discussed [here](https://discourse.llvm.org/t/rfc-introduce-sentinel-pointer-value-to-datalayout/85265/17) / in https://github.com/llvm/llvm-project/pull/131557?

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