[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