vect: Allow reduc_index != 1 for COND_OPs.

vect: Allow reduc_index != 1 for COND_OPs.

In PR112406 Tamar found another problem with COND_OP reductions.
I wrongly assumed that the reduction variable will always remain in
operand 1, just as we create the COND_OP in ifcvt.  But of course,
addition being commutative, we are free to swap operand 1 and 2 and we
end up with e.g.

 _ifc__60 = .COND_ADD (_2, _6, MADPictureC1_lsm.10_25, MADPictureC1_lsm.10_25);

which does not pass the asserts I put in place.

This patch removes this restriction and allows the reduction index to be
2 as well.

gcc/ChangeLog:

	PR middle-end/112406

	* tree-vect-loop.cc (vectorize_fold_left_reduction): Allow
	reduction index != 1.
	(vect_transform_reduction): Handle reduction index != 1.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr112406-2.c: New test.