tree-optimization/113895 - copy_reference_ops_from_ref vs. bitfields

The recent enhancement to discover constant array indices by range
info used by get_ref_base_and_extent doesn't work when the outermost
component reference is to a bitfield because we track the running
offset in the reference ops as bytes.  The following does as
ao_ref_init_from_vn_reference and recovers that manually, tracking
the offset for the purpose of discovering the constant array index
in bits instead.

	PR tree-optimization/113895
	* tree-ssa-sccvn.cc (copy_reference_ops_from_ref): Track
	offset to discover constant array indices in bits, handle
	COMPONENT_REF to bitfields.

	* gcc.dg/torture/pr113895-1.c: New testcase.