TARGET_INSN_COST gives us a better control over the instruction costs
than classical RTX_COSTS. A simple cost scheme is in place for the
time being, when optimizing for size, the cost is given by the
instruction length. When optimizing for speed, the cost is 1 for any
recognized instruction, and 2 for any load/store instruction. The
latter one can be overwritten by using cost attribute for an
instruction. Due to this change, we need to update also a number of
instruction patterns with a new predicate to better reflect the costs.