tree-optimization/115669 - fix SLP reduction association

tree-optimization/115669 - fix SLP reduction association

The following avoids associating a reduction path as that might
get STMT_VINFO_REDUC_IDX out-of-sync with the SLP operand order.
This is a latent issue with SLP reductions but now easily exposed
as we're doing single-lane SLP reductions.

When we achieved SLP only we can move and update this meta-data.

	PR tree-optimization/115669
	* tree-vect-slp.cc (vect_build_slp_tree_2): Do not reassociate
	chains that participate in a reduction.

	* gcc.dg/vect/pr115669.c: New testcase.