Update Dispatch Issue Unit to aid performance

[FinnWilkinson]

With logic updated in PR #431 to fix the unfair allocation done by the DI unit (it would stall if the first available RS was full / had reached its dispatch rate), the performance has degraded becasue of this fix.

Work should be done to overhaul the implementation of reservation stations and how the DI unit interacts with the port allocator to improve performance.