Don't calculate IPC scaling using stats["IPC"].mean()

[Jonathan3145]

Instead use something like

(stats["Useful Instructions"].sum() / stats["Useful Cycles"].sum())/
(self._stats_dict[ref_key].stats["Useful Instructions"].sum()/self._stats_dict[ref_key].stats["Useful Cycles"].sum())

[ptooley]

Doing it this way ensures that the IPC is weighted by instructions per thread rather than being indifferent to relative workload