Rewrite bounds_of_var_in_loop() to use ranges.

Rewrite bounds_of_var_in_loop() to use ranges.

Little by little, bounds_of_var_in_loop() has grown into an
unmaintainable mess.  This patch rewrites the code to use the relevant
APIs as well as refactor it to make it more readable.

gcc/ChangeLog:

	* gimple-range-fold.cc (tree_lower_bound): Delete.
	(tree_upper_bound): Delete.
	(vrp_val_max): Delete.
	(vrp_val_min): Delete.
	(fold_using_range::range_of_ssa_name_with_loop_info): Call
	range_of_var_in_loop.
	* vr-values.cc (valid_value_p): Delete.
	(fix_overflow): Delete.
	(get_scev_info): New.
	(bounds_of_var_in_loop): Refactor into...
	(induction_variable_may_overflow_p): ...this,
	(range_from_loop_direction): ...and this,
	(range_of_var_in_loop): ...and this.
	* vr-values.h (bounds_of_var_in_loop): Delete.
	(range_of_var_in_loop): New.