tree-optimization/106892 - avoid invalid pointer association in predcom

tree-optimization/106892 - 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/106892
	* tree-predcom.c (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.

(cherry picked from commit a8b0b13da7379feb31950a9d2ad74b98a29c547f)