Get an instruction from the floating-point operation queue. If there is an
empty reservation station for an appropriate FU, sent the opcode and
destination register to the reservation station.
If the operands are in registers, send them too.
If an operand is not yet in a register, it must be being computed by some active
FU. Send the identifier tag for the FU to the reservation station instead.
(To do this, the registers are tagged with the FU which will provide their
value).