ipa/116296 - avoid overflow in modref_access_node::contains

The following aovids overflow when scaling the param offset
difference by BITS_PER_UNIT by using poly_offset_int instead of
poly_int64 for the computations.

	PR ipa/116296
	* ipa-modref-tree.cc (modref_access_node::contains): Use
	poly_offset_int for the param offset difference and the
	overlap computation.

	* gcc.dg/torture/pr116296.c: New testcase.