tree-optimization/100923 - re-do VN with contextual PTA info fix

tree-optimization/100923 - re-do VN with contextual PTA info fix

The following implements the gist of the PR100923 fix in a leaner
(and more complete) way by realizing that all ao_ref_init_from_vn_reference
uses need to have an SSA name in the base valueized with availability
in mind.  Instead of re-valueizing the whole chain of operands we can
simply only and always valueize the SSA name we put in the base.

This handles also two omitted places in vn_reference_lookup_3.

	PR tree-optimization/100923
	* tree-ssa-sccvn.cc (ao_ref_init_from_vn_reference): Valueize
	base SSA_NAME.
	(vn_reference_lookup_3): Adjust vn_context_bb around calls
	to ao_ref_init_from_vn_reference.
	(vn_reference_lookup_pieces): Revert original PR100923 fix.
	(vn_reference_lookup): Likewise.