MSP430: Implement TARGET_RTX_COSTS

Costs of MSP430 instructions are mostly just a function of the type and
number of operands; knowledge of the specific instruction often
isn't required to calculate the cost.
In these cases, TARGET_RTX_COSTS just needs to examine the operands to
calculate the cost of the expression.

For more complicated operations where library helper functions are
required, if the cost cannot be accurately calculated, it is estimated
and disparaged relative to the cost of a single instruction.

gcc/ChangeLog:

	* config/msp430/msp430.c (use_helper_for_const_shift): Add forward
	declaration.
	Remove unused argument.
	(struct msp430_multlib_costs): New struct.
	(msp430_is_mem_indirect): New function.
	(msp430_costs): Likewise.
	(msp430_shift_costs): Likewise.
	(msp430_muldiv_costs): Likewise.
	(msp430_get_inner_dest_code): Likewise.
	(msp430_single_op_cost): Likewise.
	(msp430_rtx_costs): Rewrite from scratch.
	(msp430_expand_shift): Adjust use_helper_for_const_shift call.