Function fast_cost_lut_regs 

pub fn fast_cost_lut_regs(
    opcode_byte: u8,
    args: &Args,
    pc: usize,
    code: &[u8],
    bitmask: &[u8],
    ra: u8,
    rb: u8,
    rd: u8,
    mem_cycles: u8,
) -> FastCost

Like fast_cost_lut but takes pre-extracted register bytes to avoid re-reading code[pc+1] and code[pc+2] (already decoded by the caller).