[arm] Early split subdi3

[arm] Early split subdi3

This patch adds early splitting of subdi3 so that the individual
operations can be seen by the optimizers, particuarly combine.  This
should allow us to do at least as good a job as previously, but with
far fewer patterns in the machine description.

This is just the initial patch to add the early splitting.  The
cleanups will follow later.

A special trick is used to handle the 'reverse subtract and compare'
where a register is subtracted from a constant.  The natural
comparison

    (COMPARE (const) (reg))

is not canonical in this case and combine will never correctly
generate it (trying to swap the order of the operands.  To handle this
we write the comparison as

    (COMPARE (NOT (reg)) (~const)),

which has the same result for EQ, NE, LTU, LEU, GTU and GEU, which are
all the cases we are really interested in here.

Finally, we delete the negdi2 pattern.  The generic expanders will use
our new subdi3 expander if this pattern is missing and that can handle
the negate case just fine.

	* config/arm/arm-modes.def (CC_RSB): New CC mode.
	* config/arm/predicates.md (arm_borrow_operation): Handle CC_RSBmode.
	* config/arm/arm.c (arm_select_cc_mode): Detect when we should
	return CC_RSBmode.
	(maybe_get_arm_condition_code): Handle CC_RSBmode.
	* config/arm/arm.md (subsi3_carryin): Make this pattern available to
	expand.
	(subdi3): Rewrite to early-expand the sub-operations.
	(rsb_im_compare): New pattern.
	(negdi2): Delete.
	(negdi2_insn): Delete.
	(arm_negsi2): Correct type attribute to alu_imm.
	(negsi2_0compare): New insn pattern.
	(negsi2_carryin): New insn pattern.

From-SVN: r277169