tree-optimization/96920 - another ICE when vectorizing nested cycles

tree-optimization/96920 - another ICE when vectorizing nested cycles

This refines the previous fix for PR96698 by re-doing how and where
we arrange for setting vectorized cycle PHI backedge values.

2020-09-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96698
	PR tree-optimization/96920
	* tree-vectorizer.h (loop_vec_info::reduc_latch_defs): Remove.
	(loop_vec_info::reduc_latch_slp_defs): Likewise.
	* tree-vect-stmts.c (vect_transform_stmt): Remove vectorized
	cycle PHI latch code.
	* tree-vect-loop.c (maybe_set_vectorized_backedge_value): New
	helper to set vectorized cycle PHI latch values.
	(vect_transform_loop): Walk over all PHIs again after
	vectorizing them, calling maybe_set_vectorized_backedge_value.
	Call maybe_set_vectorized_backedge_value for each vectorized
	stmt.  Remove delayed update code.
	* tree-vect-slp.c (vect_analyze_slp_instance): Initialize
	SLP instance reduc_phis member.
	(vect_schedule_slp): Set vectorized cycle PHI latch values.

	* gfortran.dg/vect/pr96920.f90: New testcase.
	* gcc.dg/vect/pr96920.c: Likewise.