tree-optimization/98685 - fix placement of extern converts

Avoid advancing to the next stmt when inserting at region boundary
and deal with a vector def being not the only child.

2021-01-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98685
	* tree-vect-slp.c (vect_schedule_slp_node): Refactor handling
	of vector extern defs.

	* gcc.dg/vect/bb-slp-pr98685.c: New testcase.