tree-optimization/104825 - guard modref query

The following makes sure to guard the modref query in VN on a
pointer typed argument.

2022-03-08  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104825
	* tree-ssa-sccvn.cc (visit_reference_op_call): Properly
	guard modref get_ao_ref on a pointer typed argument.

	* gcc.dg/torture/pr104825.c: New testcase.