middle-end/108086 - avoid quadraticness in copy_edges_for_bb

middle-end/108086 - avoid quadraticness in copy_edges_for_bb

For the testcase in PR108086 it's visible that we split blocks
multiple times when inlining and that causes us to adjust the
block tail stmt BBs multiple times, once for each split.  The
fix is to walk backwards and split from the tail instead.

For a reduced testcase this improves compile-time at -O by 4%.

	PR middle-end/108086
	* tree-inline.cc (copy_edges_for_bb): Walk stmts backwards for
	splitting the block to avoid quadratic behavior with setting
	stmts BB on multliple splits.