process transitive inferred ranges in pre_fold_stmt.

The subst_and_fold engine can perform some folding activity before
calling fold_stmt, so do this work in pre_fold_stmt instead.

	* tree-vrp.cc (rvrp_folder::rvrp_folder): Init m_last_bb_stmt.
	(rvrp_folder::pre_fold_bb): Set m_last_bb_stmt.
	(rvrp_folder::pre_fold_stmt): Check for transitive inferred ranges.
	(rvrp_folder::fold_stmt): Check in pre_fold_stmt instead.