Unify Identity range operator

Move the declaration of the class to the range-op-mixed header, add the
floating point prototypes as well, and use it in the new unified table.

	* range-op-float.cc (foperator_identity): Remove.  Move prototypes
	to range-op-mixed.h
	(operator_identity::fold_range): Rename from foperator_identity.
	(operator_identity::op1_range): Ditto.
	(float_table::float_table): Remove fop_identity.
	* range-op-mixed.h (class operator_identity): Combined from integer
	and float files.
	* range-op.cc (op_identity): New object.
	(unified_table::unified_table): Add op_identity.
	(class operator_identity): Move to range-op-mixed.h.
	(integral_table::integral_table): Remove identity.
	(pointer_table::pointer_table): Remove identity.