tree-optimization/107617 - big-endian .LEN_STORE VN

tree-optimization/107617 - big-endian .LEN_STORE VN

The following fixes a mistake in interpreting .LEN_STORE definitions
during value-numbering when in big-endian mode.  We cannot offset
the encoding of the RHS but instead encode to an offsetted position
which is then treated correctly by the endian aware copying code.

	PR tree-optimization/107617
	* tree-ssa-sccvn.cc (vn_walk_cb_data::push_partial_def):
	Handle negative pd.rhs_off.
	(vn_reference_lookup_3): Properly provide pd.rhs_off
	for .LEN_STORE on big-endian targets.