Constrain PHI handling in -Wuse-after-free [PR104232].

Constrain PHI handling in -Wuse-after-free [PR104232].

Resolves:
PR middle-end/104232 - spurious -Wuse-after-free after conditional free

gcc/ChangeLog:

	PR middle-end/104232
	* gimple-ssa-warn-access.cc (pointers_related_p): Add argument.
	Handle PHIs.  Add a synonymous overload.
	(pass_waccess::check_pointer_uses): Call pointers_related_p.

gcc/testsuite/ChangeLog:

	PR middle-end/104232
	* g++.dg/warn/Wuse-after-free4.C: New test.
	* gcc.dg/Wuse-after-free-2.c: New test.
	* gcc.dg/Wuse-after-free-3.c: New test.