tree-optimization/121370 - avoid UB in building a CHREC

tree-optimization/121370 - avoid UB in building a CHREC

When there is obvious UB involved in the process of re-associating
a series of IV increments to build up a CHREC, fail.  This catches
a few degenerate cases where SCEV introduces UB with its inherent
re-associating of IV increments.

	PR tree-optimization/121370
	* tree-scalar-evolution.cc (scev_dfs::add_to_evolution_1):
	Avoid UB integer overflow in accumulating CHREC_RIGHT.

	* gcc.dg/torture/pr121370.c: New testcase.