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

[Shilei Tian via llvm-commits]

shiltian wrote:

> It is. Making `null` not be the all-zeroes value in all address spaces is a proposed change, but implementing that takes a lot more than adding one DataLayout method. The status quo is that null is always the all-zeroes value.

Yes and in fact the data layout change would be the last step after we have proper representation of the constant null ptr. I have something WIP locally but got stuck by the aggregate type. Just had some discussion with @arsenm this morning so will pick it up later.

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