tree-optimization/103816 - detect offset overflow in SLP group analysis

tree-optimization/103816 - detect offset overflow in SLP group analysis

This makes sure to detect overflow when computing DR_GROUP_GAP
and DR_GROUP_SIZE more thoroughly so artificial testcases like the
added one are not fooling the existing check.

2022-01-05  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/103816
	* tree-vect-data-refs.c (vect_analyze_group_access_1): Also
	check DR_GROUP_GAP compute for overflow and representability.

	* gcc.dg/torture/pr103816.c: New testcase.