[llvm-exegesis] Optimize ToProcess in dbScan
[llvm-exegesis] Optimize ToProcess in dbScan

Summary:
Use `vector<char> Added + vector<size_t> ToProcess` to replace `SetVector ToProcess`

We also check `Added[P]` to enqueueing a point more than once, which
also saves us a `ClusterIdForPoint_[Q].isUndef()` check.

Reviewers: courbet, RKSimon, gchatelet, john.brawn, lebedev.ri

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54442

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@349136 91177308-0d34-0410-b5e6-96231b3b80d8

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