tree-optimization/98855 - fix some vectorizer cost issues

tree-optimization/98855 - fix some vectorizer cost issues

This fixes us not costing vectorized bswap for SLP as well as
avoiding biasing to the vectorized side when costing single-argument
PHIs.  Instead we assume coalescing here and cost them with zero cost
for both the scalar and vectorized code.

This doesn't fix the PR on its own.

2021-02-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98855
	* tree-vect-loop.c (vectorizable_phi): Do not cost
	single-argument PHIs.
	* tree-vect-slp.c (vect_bb_slp_scalar_cost): Likewise.
	* tree-vect-stmts.c (vectorizable_bswap): Also perform
	costing for SLP operation.