tree-optimization/119960 - fix and guard get_later_stmt

The following makes get_later_stmt handle stmts from different
basic-blocks in the case they are orderd and otherwise asserts.

	* tree-vectorizer.h (get_later_stmt): Robustify against
	stmts in different BBs, assert when they are unordered.

(cherry picked from commit a6cfde60d8c744b31b147022e797bbcc371ae092)