tree-optimization/98854 - avoid some PHI BB vectorization

This avoids cases of PHI node vectorization that just causes us
to insert vector CTORs inside loops for values only required
outside of the loop.

2021-01-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98854
	* tree-vect-slp.c (vect_build_slp_tree_2): Also build
	PHIs from scalars when the number of CTORs matches the
	number of children.

	* gcc.dg/vect/bb-slp-pr98854.c: New testcase.