tree-optimization/106722 - avoid invalid pointer association in predcom

tree-optimization/106722 - avoid invalid pointer association in predcom

When predictive commoning builds a reference for iteration N it
prematurely associates a constant offset into the MEM_REF offset
operand which can be invalid if the base pointer then points
outside of an object which alias-analysis does not consider valid.

	PR tree-optimization/106722
	* tree-predcom.cc (ref_at_iteration): Do not associate the
	constant part of the offset into the MEM_REF offset
	operand, across a non-zero offset.

	* gcc.dg/torture/pr106892.c: New testcase.