tree-optimization/127473 - avoid starting a group with unvectorizable load

The following avoids starting a group during SLP discovery with
an unvectorizable load by refactoring the BB specific not grouped
load handling to apply before selecting the first match.

	PR tree-optimization/127473
	* tree-vect-slp.cc (vect_build_slp_tree_3): Duplicate the BB
	case of rejecting a not grouped load early to avoid assiging
	the first_match to it.