Only issue warning for subtraction involving null pointers on live code paths

Summary:
Change the warning produced for subtraction from (or with) a null pointer
to only be produced when the code path is live.
https://github.com/llvm/llvm-project/issues/54570

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: anarazel (Andres Freund)
Differential Revision: https://reviews.llvm.org/D126816