tree-optimization/95539 - fix SLP_TREE_REPRESENTATIVE vs. dr_info

tree-optimization/95539 - fix SLP_TREE_REPRESENTATIVE vs. dr_info

This fixes a disconnect between the stmt_info used for dr_info
analysis and the one in SLP_TREE_REPRESENTATIVE with a temporary
workaround.

2020-06-05  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95539
	* tree-vect-data-refs.c
	(vect_slp_analyze_and_verify_instance_alignment): Use
	SLP_TREE_REPRESENTATIVE for the data-ref check.
	* tree-vect-stmts.c (vectorizable_load): Reset stmt_info
	back to the first scalar stmt rather than the
	SLP_TREE_REPRESENTATIVE to match previous behavior.

	* gcc.dg/vect/pr95539.c: New testcase.