[llvm-mca] Speed up the computation of the wait/ready/issued sets in the Scheduler.

This patch is a follow-up to r338702.

We don't need to use a map to model the wait/ready/issued sets. It is much more
efficient to use a vector instead.

This patch gives us an average 7.5% speedup (on top of the ~12% speedup obtained
after r338702).

llvm-svn: 338883