mi-sched: smooth out the cyclicpath heuristic.

Arnold's idea.

I generally try to avoid stateful heuristics because it can make
debugging harder. However, we need a way to prevent the latency
priority from dominating, and it somewhat makes sense to schedule
aggressively for latency only within an issue group.

Swift in particular likes this, and it doesn't hurt anyone else:
| Benchmarks/MiBench/consumer-lame              |  10.39% |
| Benchmarks/Misc/himenobmtxpa                  |   9.63% |

llvm-svn: 190360