tree-optimization/93381 fix integer offsetting in points-to analysis

We were incorrectly assuming a merge operation is conservative enough
for not explicitely handled operations but we also need to consider
offsetting within fields when field-sensitive analysis applies.

2020-01-22  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93381
	* tree-ssa-structalias.c (find_func_aliases): Assume offsetting
	throughout, handle all conversions the same.

	* gcc.dg/torture/pr93381.c: New testcase.