tree-optimization/97678 - fix SLP induction epilogue vectorization

tree-optimization/97678 - fix SLP induction epilogue vectorization

This restores not tracking SLP nodes for induction initial values
in not nested context because this interferes with peeling and
epilogue vectorization.

2020-11-03  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97678
	* tree-vect-slp.c (vect_build_slp_tree_2): Do not track
	the initial values of inductions when not nested.
	* tree-vect-loop.c (vectorizable_induction): Look at
	PHI node initial values again for SLP and not nested
	inductions.  Handle LOOP_VINFO_MASK_SKIP_NITERS and cost
	invariants.

	* gcc.dg/vect/pr97678.c: New testcase.