Check for casts becoming UNDEFINED.

Check for casts becoming UNDEFINED.

In various situations a cast that is ultimately unreahcable may produce
an UNDEFINED result, and we can't check the bounds in this case.

	PR tree-optimization/120277
	gcc/
	* range-op-ptr.cc (operator_cast::fold_range): Check if the cast
	if UNDEFINED before setting bounds.

	gcc/testsuite/
	* gcc.dg/pr120277.c: New.