[llvm-exegesis] InstructionBenchmarkClustering::rangeQuery(): use...

[llvm-exegesis] InstructionBenchmarkClustering::rangeQuery(): use llvm::SmallVector<size_t, 0> for storage.

Summary:
Old: (D54383)
```
 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):

       9098.781978      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.16% )
...
            9.1015 +- 0.0148 seconds time elapsed  ( +-  0.16% )
```
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):

       8553.352480      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.12% )
...
            8.5539 +- 0.0105 seconds time elapsed  ( +-  0.12% )
```
So another -6%.
That is because the `SmallVector` **doubles** it size when reallocating, which is great here,
since we can't `reserve()` since we can't know how many `Neighbors` we will have.

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

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54388

llvm-svn: 347200