tree-optimization/96698 - fix ICE when vectorizing nested cycles

tree-optimization/96698 - fix ICE when vectorizing nested cycles

This fixes vectorized PHI latch edge updating and delay it until
all of the loop is code generated to deal with the case that the
latch def is a PHI in the same block.

2020-08-26  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96698
	* tree-vectorizer.h (loop_vec_info::reduc_latch_defs): New.
	(loop_vec_info::reduc_latch_slp_defs): Likewise.
	* tree-vect-stmts.c (vect_transform_stmt): Only record
	stmts to update PHI latches from, perform the update ...
	* tree-vect-loop.c (vect_transform_loop): ... here after
	vectorizing those PHIs.
	(info_for_reduction): Properly handle non-reduction PHIs.

	* gcc.dg/vect/pr96698.c: New testcase.