tree-optimization/104240 - SLP discovery with swapped comparisons

The following extends SLP discovery to handle swapped operands
in comparisons.

2022-05-02  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104240
	* tree-vect-slp.cc (op1_op0_map): New.
	(vect_get_operand_map): Handle compares.
	(vect_build_slp_tree_1): Support swapped operands for
	tcc_comparison.

	* gcc.dg/vect/bb-slp-pr104240.c: New testcase.