Avoid PHI - PHI recurrence in vectorization

Avoid PHI - PHI recurrence in vectorization

The reported regression of libgomp loop-14.C shows that there isn't
generally a good reliable place to insert the permute upfront so
the following simply restricts recurrence vectorization to the cases
where the latch value isn't defined by a PHI.

	* tree-vect-loop.cc (vect_phi_first_order_recurrence_p):
	Disallow latch PHI defs.
	(vectorizable_recurr): Revert previous change.