tree-optimization/121509 - failure to detect unvectorizable loop

tree-optimization/121509 - failure to detect unvectorizable loop

With the hybrid stmt detection no longer working as a gate-keeper
to detect unhandled stmts we have to, and can, detect those earlier.
The appropriate place is vect_mark_stmts_to_be_vectorized where
for trivially relevant PHIs we can stop analyzing when the PHI
wasn't classified as a known def during vect_analyze_scalar_cycles.

	PR tree-optimization/121509
	* tree-vect-stmts.cc (vect_mark_stmts_to_be_vectorized):
	Fail early when we detect a relevant but not handled PHI.

	* gcc.dg/vect/pr121509.c: New testcase.