adjust BB SLP build from scalars heuristics

adjust BB SLP build from scalars heuristics

We can end up with { _1, 1.0 } * { 3.0, _2 } which isn't really
profitable.  The following adjusts things so we reject more than
one possibly expensive (non-constant and not uniform) vector CTOR
and instead build a CTOR for the scalar operation results.

This also moves a check in vect_get_and_check_slp_defs to a better
place.

2020-10-14  Richard Biener  <rguenther@suse.de>

	* tree-vect-slp.c (vect_get_and_check_slp_defs): Move
	check for duplicate/interleave of variable size constants
	to a place done once and early.
	(vect_build_slp_tree_2): Adjust heuristics when to build
	a BB SLP node from scalars.