Improve BB vectorization opt-info

The following makes us more correctly print the used vector size
when doing BB vectorization and also print all involved SLP graph
roots, not just the random one we ended up picking as leader.
In particular the last bit improves diffing opt-info between
different GCC revs but it also requires some testsuite adjustments.

	* tree-vect-slp.cc (vect_slp_region): Provide opt-info for all SLP
	subgraph entries.  Dump the used vector size based on the
	SLP subgraph entry root vector type.

	* g++.dg/vect/slp-pr87105.cc: Adjust.
	* gcc.dg/vect/bb-slp-17.c: Likewise.
	* gcc.dg/vect/bb-slp-20.c: Likewise.
	* gcc.dg/vect/bb-slp-21.c: Likewise.
	* gcc.dg/vect/bb-slp-22.c: Likewise.
	* gcc.dg/vect/bb-slp-subgroups-2.c: Likewise.