tree-optimization/126861 - ICE with SLP schedule at explicit region start

We cannot use a random stmt at region start as marker as that might
eventually be a control stmt.  Instead use NULL to mark region
start as documented.

	PR tree-optimization/126861
	* tree-vect-slp.cc (vect_schedule_slp_node): Use NULL for
	scheduling at region start.

	* g++.dg/vect/slp-pr126861.cc: New testcase.