Avoid using POINTER_DIFF_EXPR for overlap checks [PR119399]

Avoid using POINTER_DIFF_EXPR for overlap checks [PR119399]

In r10-4803-g8489e1f45b50600c I'd used POINTER_DIFF_EXPR to subtract
the two pointers involved in an overlap test.  I'm not sure whether
I'd specifically chosen that over MINUS_EXPR or not; if so, the only
reason I can think of is that it is probably faster on targets with
PSImode pointers.  Regardless, as the PR points out, subtracting
unrelated pointers using POINTER_DIFF_EXPR is undefined behaviour.

gcc/
	PR tree-optimization/119399
	* tree-data-ref.cc (create_waw_or_war_checks): Use a MINUS_EXPR
	on two converted pointers, rather than converting a POINTER_DIFF_EXPR
	on the pointers.

gcc/testsuite/
	PR tree-optimization/119399
	* gcc.dg/vect/pr119399.c: New test.

(cherry picked from commit 4c8c373495d7d863dfb7102726ac3b4b41685df4)