Improve uninitialized warning with value range info

Function use_pred_not_overlap_with_undef_path_pred of
pass_late_warn_uninitialized
checks if predicate of variable use overlaps with predicate of undefined
control flow path.
For now, it only checks ssa_var comparing against constant, this can be
improved where
ssa_var compares against another ssa_var with value range info, as described in
comment:

+         /* Check value range info of rhs, do following transforms:
+              flag_var < [min, max]  ->  flag_var < max
+              flag_var > [min, max]  ->  flag_var > min
+
+            We can also transform LE_EXPR/GE_EXPR to LT_EXPR/GT_EXPR:
+              flag_var <= [min, max] ->  flag_var < [min, max+1]
+              flag_var >= [min, max] ->  flag_var > [min-1, max]
+            if no overflow/wrap.  */

gcc/

	* tree-ssa-uninit.c (find_var_cmp_const): New function.
	(use_pred_not_overlap_with_undef_path_pred): Call above.