tree-optimization/95897 - fix fold-left SLP reduction insert place

tree-optimization/95897 - fix fold-left SLP reduction insert place

This fixes computation of the insertion place for fold-left SLP
reductions where the PHIs do not have vectorized stmts.  The
SLP representation isn't perfect here thus the following.

2020-06-26  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95897
	* tree-vectorizer.h (vectorizable_induction): Remove
	unused gimple_stmt_iterator * parameter.
	* tree-vect-loop.c (vectorizable_induction): Likewise.
	(vect_analyze_loop_operations): Adjust.
	* tree-vect-stmts.c (vect_analyze_stmt): Likewise.
	(vect_transform_stmt): Likewise.
	* tree-vect-slp.c (vect_schedule_slp_instance): Adjust
	for fold-left reductions, clarify existing reduction case.

	* gcc.dg/vect/pr95897.c: New testcase.