tree-optimization/103029 - ensure vect loop versioning constraint on PHIs

tree-optimization/103029 - ensure vect loop versioning constraint on PHIs

PHI nodes in vectorizer loop versioning need to maintain the same
order of PHI arguments to not disturb SLP discovery.  The following
adds an assertion and mitigation in case loop versioning breaks this
which happens more often after the recent reorg.

2021-11-02  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103029
	* tree-vect-loop-manip.c (vect_loop_versioning): Ensure
	the PHI nodes in the loop maintain their original operand
	order.