tree-optimization/95866 - avoid vectorizing uniform SLP subgraphs

tree-optimization/95866 - avoid vectorizing uniform SLP subgraphs

This avoids vectorizing SLP subgraphs that just compute uniform
operations on all-same operands.  That fixes the less interesting
(but most embarrasing) part of the testcase in the PR.  On the
way it also fixed a missing matches[0] reset in the last
refactoring touching that place.

2020-06-24  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95866
	* tree-vect-slp.c (vect_slp_tree_uniform_p): New.
	(vect_build_slp_tree_2): Properly reset matches[0],
	ignore uniform constants.

	* gcc.target/i386/pr95866-1.c: New testcase.