Make each def a new equivalency record.

Make each def a new equivalency record.

Create a new equivalency set at each def point killing any equivalencies
coming into the block from back edges.  Do not add equivalences for PHI
arguments defined in this block.

	* value-relation.cc (equiv_oracle::register_initial_def): New.
	(equiv_oracle::register_relation): Call register_initial_def.
	(equiv_oracle::add_equiv_to_block): New.  Split register_relation.
	(relation_oracle::register_stmt): Check def block of PHI arguments.
	* value-relation.h (equiv_oracle): Add new prototypes.