Provide some context to folding via ranger.

Provide some context to folding via ranger.

Provide an internal mechanism to supply context to range_of_expr for calls
to ::fold_stmt.

	* gimple-range.cc (gimple_ranger::gimple_ranger): Initialize current_bb.
	(gimple_ranger::range_of_expr): Pick up range_on_entry when there is
	no explcit context and current_bb is set.
	(gimple_ranger::fold_stmt): New.
	* gimple-range.h (current_bb, fold_stmt): New.
	* tree-vrp.c (rvrp_folder::fold_stmt): Call ranger's fold_stmt.