[llvm-exegesis] InstructionBenchmarkClustering::dbScan(): replace std::vector<> with...

[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