tree-optimization/103237 - avoid vectorizing unhandled double reductions

tree-optimization/103237 - avoid vectorizing unhandled double reductions

Double reductions which have multiple LC PHIs in the inner loop
are not handled correctly during transformation since those PHIs
are not properly classified as reduction.  The following disables
vectorizing them.

2021-11-15  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103237
	* tree-vect-loop.c (vect_is_simple_reduction): Fail for
	double reductions with multiple inner loop LC PHI nodes.

	* gcc.dg/torture/pr103237.c: New testcase.

(cherry picked from commit 220bd61874cf114667b44f9ded76ed0639eb278b)