[arm] Correctly cost addition with a carry-in

[arm] Correctly cost addition with a carry-in

The cost routine for Arm and Thumb2 was not recognising the idioms that
describe the addition with carry, this results in the instructions
appearing more expensive than they really are, which occasionally can lead
to poor choices by combine.  Recognising all the possible variants is
a little trickier than normal because the expressions can become complex
enough that this is no single canonical from.

	* config/arm/arm.c (strip_carry_operation): New function.
	(arm_rtx_costs_internal, case PLUS): Handle addtion with carry-in
	for SImode.

From-SVN: r277172