[IRBuilder] Use ptrtoaddr in CreatePtrDiff() (#181855)
Make CreatePtrDiff() emit the pattern `ptrtoaddr(p1)-ptrtoaddr(p2)`.
This makes a few changes:
* The return type is now the address type instead of hardcoded to i64.
I've adjusted callers to deal with this where they didn't already.
* Don't use `ConstantExpr::getSizeOf()` and instead get the actual size
from DataLayout. These sizeof expressions will be removed as part of the
ptradd migration.
* Add a convenience overload without the element type, for the case
where you want a pure pointer difference.
I also adjusted some OpenMP code to consistently use zext for sizes, as
I had issues updating the test coverage otherwise (as we ended up
randomly picking zext or sext depending on the exact code path).
