[llvm-exegesis] Move InstructionBenchmarkClustering::isNeighbour() into header

Summary:
Old: (D54390)
```
 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):

       7432.421721      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.15% )
...
            7.4336 +- 0.0115 seconds time elapsed  ( +-  0.15% )
```
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):

       6569.936144      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.22% )
...
            6.5711 +- 0.0143 seconds time elapsed  ( +-  0.22% )
```
And another -12%. You'd think it would be `inline`d anyway, but no! :)

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

Reviewed By: courbet, MaskRay

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54393

llvm-svn: 347203