fixup BB vectorization constant generation place

fixup BB vectorization constant generation place

This adjusts the way we compute the stmt insert location for
invariants in BB vectorization context to deal with eventually
sharing invariant SLP nodes for multiple uses.  We can no longer
use a single use stmt location then but there's a simple way out.

2020-05-18  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_slp_bb): Start after labels.
	(vect_get_constant_vectors): Really place init stmt after scalar defs.
	* tree-vect-stmts.c (vect_init_vector_1): Insert before
	region begin.