tree-optimization/114048 - ICE in copy_reference_ops_from_ref

The following adds another omission to the assert verifying we're
not running into spurious off == -1.

	PR tree-optimization/114048
	* tree-ssa-sccvn.cc (copy_reference_ops_from_ref): MEM_REF
	can also produce -1 off.

	* gcc.dg/torture/pr114048.c: New testcase.