Harden SLP reduction support wrt STMT_VINFO_REDUC_IDX

Harden SLP reduction support wrt STMT_VINFO_REDUC_IDX

The following makes sure that for a SLP reductions all lanes have
the same STMT_VINFO_REDUC_IDX.  Once we move that info and can adjust
it we can implement swapping.  It also makes the existing protection
against operand swapping trigger for all stmts participating in a
reduction, not just the final one marked as reduction-def.

	* tree-vect-slp.cc (vect_build_slp_tree_1): Compare
	STMT_VINFO_REDUC_IDX.
	(vect_build_slp_tree_2): Prevent operand swapping for
	all stmts participating in a reduction.