Unify MINUS_EXPR range operator

Unify MINUS_EXPR 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_minus): Remove.  Move prototypes
	to range-op-mixed.h
	(operator_minus::fold_range): Rename from foperator_minus.
	(operator_minus::op1_range): Ditto.
	(operator_minus::op2_range): Ditto.
	(operator_minus::rv_fold): Ditto.
	(float_table::float_table): Remove MINUS_EXPR.
	* range-op-mixed.h (class operator_minus): Combined from integer
	and float files.
	* range-op.cc (op_minus): New object.
	(unified_table::unified_table): Add MINUS_EXPR.
	(class operator_minus): Move to range-op-mixed.h.
	(integral_table::integral_table): Remove MINUS_EXPR.
	(pointer_table::pointer_table): Remove MINUS_EXPR.