[llvm-exegesis] InstructionBenchmarkClustering::dbScan(): replace std…

…::vector<> with std::deque<> in llvm::SetVector<>

Summary:
Old: (D54388)
```
 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html' (10 runs):

       8606.323981      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.11% )
...
           8.60773 +- 0.00978 seconds time elapsed  ( +-  0.11% )
```
New:
```
 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=100000 -benchmarks-file=/tmp/benchmarks.yaml -analysis-inconsistencies-output-file=/tmp/clusters.html' (10 runs):

       7971.403653      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.14% )
...
            7.9728 +- 0.0113 seconds time elapsed  ( +-  0.14% )
```
Another -~7%.

Reviewers: courbet, MaskRay, RKSimon, gchatelet, john.brawn

Reviewed By: courbet, RKSimon

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54389

llvm-svn: 347201

llvm/tools/llvm-exegesis/lib/Clustering.cpp

@@ -121,7 +121,7 @@ void InstructionBenchmarkClustering::dbScan(const size_t MinPts) {
     CurrentCluster.PointIndices.push_back(P);
 
     // Process P's neighbors.
-    llvm::SetVector<size_t> ToProcess;
+    llvm::SetVector<size_t, std::deque<size_t>> ToProcess;
     ToProcess.insert(Neighbors.begin(), Neighbors.end());
     while (!ToProcess.empty()) {
       // Retrieve a point from the set.