Swap operands during SLP discovery for mismatching STMT_VINFO_REDUC_IDX

Swap operands during SLP discovery for mismatching STMT_VINFO_REDUC_IDX

When we are unlucky operand canonicalization can end up presenting
us with different order, making a possible SLP reduction group
not match up.  The following allows swapping operands in this case.

	* tree-vect-slp.cc (vect_get_operand_map): Handle commutative
	operands when swapping is requested.
	(vect_build_slp_tree_1): Allow STMT_VINFO_REDUC_IDX differences
	when operand swapping makes them match and request swapping.
	(vect_build_slp_instance): Indicate we have successfully
	discovered a SLP reduction group.

	* gcc.dg/vect/slp-reduc-13.c: New testcase.

Co-authored-by: Eric Botcazou <ebotcazou@adacore.com>