tree-optimization/98550 - fix BB vect unrolling check

This fixes the check that disqualifies BB vectorization because of
required unrolling to match up with the later exact_div we do.  To
not disable the ability to split groups that do not match up
exactly with a choosen vector type this also introduces a soft-fail
mechanism to vect_build_slp_tree_1 which delays failing to after
the matches[] array is populated from other checks and only then
determines the split point according to the vector type.

2021-01-12  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98550
	* tree-vect-slp.c (vect_record_max_nunits): Check whether
	the group size is a multiple of the vector element count.
	(vect_build_slp_tree_1): When we need to fail because
	the vector type choosen causes unrolling do so lazily
	without affecting matches only at the end to guide group splitting.

	* g++.dg/opt/pr98550.C: New testcase.

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