tree-optimization/96028 - fix bogus externalizing of SLP node

tree-optimization/96028 - fix bogus externalizing of SLP node

This guards externalizing a SLP node when it fails to code generate
to actually have scalar defs we can use.  It also makes failure
to do so not fell the whole SLP instance but instead try this again
on the parent.

2020-07-02  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96028
	* tree-vect-slp.c (vect_slp_convert_to_external): Make sure
	we have scalar stmts to use.
	(vect_slp_analyze_node_operations): When analyzing a child
	failed try externalizing the parent node.