tree-optimization/109791 - simplify (unsigned)&foo - (unsigned)(&foo + o)

The following adds another variant of address difference simplification.
The utility ptr_difference_const only handles constant differences
(we also cannot code generate anything else), so exposing a possible
POINTER_PLUS_EXPR in the match and computing the difference on the
base only makes it possible to handle one case of a variable offset.
This simplifies

(unsigned long) &MEM <char[3]> [(void *)&str + 2B] - (unsigned long) (&str + (_69 + 1))

down to (1 - (unsigned long) _69) during niter analysis, allowing
ranger to eliminate a condition later and avoiding a bogus
-Wstringop-overflow diagnostic for the testcase in the PR.

	PR tree-optimization/109791
	* match.pd (minus (convert ADDR_EXPR@0) (convert (pointer_plus @1 @2))):
	New pattern.
	(minus (convert (pointer_plus @1 @2)) (convert ADDR_EXPR@0)):
	Likewise.

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