MSP430: Implement TARGET_MEMORY_MOVE_COST

MSP430: Implement TARGET_MEMORY_MOVE_COST

The cycle and size cost of a MOV instruction in different addressing
modes can be used to calculate the TARGET_MEMORY_MOVE_COST relative to
TARGET_REGISTER_MOVE_COST.

gcc/ChangeLog:

	* config/msp430/msp430.c (struct single_op_cost): New struct.
	(struct double_op_cost): Likewise.
	(TARGET_REGISTER_MOVE_COST): Don't define but add comment.
	(TARGET_MEMORY_MOVE_COST): Define to...
	(msp430_memory_move_cost): New function.
	(BRANCH_COST): Don't define but add comment.