[llvm-exegesis] InstructionBenchmarkClustering::rangeQuery(): write into...

[llvm-exegesis] InstructionBenchmarkClustering::rangeQuery(): write into llvm::SmallVectorImpl& output parameter

Summary:
I do believe this is the correct fix.
We call `rangeQuery()` *very* often. And many times it's output vector is large (tens of thousands entries), so small-size-opt won't help.

Old: (D54389)
```
 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):

       7934.528363      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.19% )
...
            7.9354 +- 0.0148 seconds time elapsed  ( +-  0.19% )
```
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):

       7383.793440      task-clock (msec)         #    1.000 CPUs utilized            ( +-  0.47% )
...
            7.3868 +- 0.0340 seconds time elapsed  ( +-  0.46% )
```
And another -7%. And that isn't even the good bit yet.

Old:
* calls to allocation functions: 2081419
* temporary allocations: 219658 (10.55%)
* bytes allocated in total (ignoring deallocations): 4.31 GB

New:
* calls to allocation functions: 1880295 (-10%)
* temporary allocations: 18758 (1%) (-91% *sic*)
* bytes allocated in total (ignoring deallocations): 545.15 MB (-88% *sic*)

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

Reviewed By: courbet, MaskRay

Subscribers: tschuett, llvm-commits

Differential Revision: https://reviews.llvm.org/D54390

llvm-svn: 347202