Make some Value_Range's explicitly integer.

Make some Value_Range's explicitly integer.

Fix some Value_Range's that we know ahead of time will be only
integers.  This avoids using the polymorphic Value_Range unnecessarily

gcc/ChangeLog:

	* gimple-ssa-warn-access.cc (check_nul_terminated_array): Make Value_Range an int_range.
	(memmodel_to_uhwi): Same
	* tree-ssa-loop-niter.cc (refine_value_range_using_guard): Same.
	(determine_value_range): Same.
	(infer_loop_bounds_from_signedness): Same.
	(scev_var_range_cant_overflow): Same.