[llvm-exegesis] Cut run time of analysis mode by another -35% (*sic*) (YamlContext...

[llvm-exegesis] Cut run time of analysis mode by another -35% (*sic*) (YamlContext::getRegNo())

Summary:

Together with the previous patch, it's an -90% improvement,
or roughly -96% improvement if you look starting with rL347204

```
$ perf stat -r 9 ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput-onefull.yaml -analysis-clusters-output-file="" -analysis-inconsistencies-output-file=/tmp/clusters-bew.html
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 14656 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-bew.html'
...
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 14656 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-bew.html'

 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput-onefull.yaml -analysis-clusters-output-file= -analysis-inconsistencies-output-file=/tmp/clusters-bew.html' (9 runs):

           1483.18 msec task-clock                #    0.999 CPUs utilized            ( +-  0.10% )
                68      context-switches          #   46.085 M/sec                    ( +- 22.62% )
                 0      cpu-migrations            #    0.000 K/sec
             11641      page-faults               # 7850.880 M/sec                    ( +-  0.62% )
        5943246799      cycles                    # 4008184.428 GHz                   ( +-  0.10% )  (83.28%)
         442869514      stalled-cycles-frontend   #    7.45% frontend cycles idle     ( +-  0.41% )  (83.29%)
        1443375663      stalled-cycles-backend    #   24.29% backend cycles idle      ( +-  0.47% )  (33.43%)
        7714006752      instructions              #    1.30  insn per cycle
                                                  #    0.19  stalled cycles per insn  ( +-  0.07% )  (50.17%)
        1977242936      branches                  # 1333472193.855 M/sec              ( +-  0.07% )  (66.79%)
          32624220      branch-misses             #    1.65% of all branches          ( +-  0.18% )  (83.34%)

           1.48438 +- 0.00143 seconds time elapsed  ( +-  0.10% )
```
```
$ perf stat -r 9 ./bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput-onefull.yaml -analysis-clusters-output-file="" -analysis-inconsistencies-output-file=/tmp/clusters-newer.html
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 14656 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-newer.html'
...
no exegesis target for x86_64-unknown-linux-gnu, using default
Parsed 14656 benchmark points
Printing sched class consistency analysis results to file '/tmp/clusters-newer.html'

 Performance counter stats for './bin/llvm-exegesis -mode=analysis -analysis-epsilon=1.0 -benchmarks-file=/tmp/benchmarks-inverse_throughput-onefull.yaml -analysis-clusters-output-file= -analysis-inconsistencies-output-file=/tmp/clusters-newer.html' (9 runs):

            963.28 msec task-clock                #    0.999 CPUs utilized            ( +-  0.37% )
                12      context-switches          #   12.695 M/sec                    ( +- 52.79% )
                 0      cpu-migrations            #    0.000 K/sec
             11599      page-faults               # 12046.971 M/sec                   ( +-  0.59% )
        3860122322      cycles                    # 4009359.596 GHz                   ( +-  0.37% )  (83.19%)
         380300669      stalled-cycles-frontend   #    9.85% frontend cycles idle     ( +-  0.34% )  (83.30%)
        1071910340      stalled-cycles-backend    #   27.77% backend cycles idle      ( +-  1.30% )  (33.51%)
        4773418224      instructions              #    1.24  insn per cycle
                                                  #    0.22  stalled cycles per insn  ( +-  0.15% )  (50.17%)
        1106990316      branches                  # 1149787979.919 M/sec              ( +-  0.11% )  (66.80%)
          23632231      branch-misses             #    2.13% of all branches          ( +-  0.18% )  (83.33%)

           0.96389 +- 0.00356 seconds time elapsed  ( +-  0.37% )
```
```
$ sha512sum /tmp/clusters-*
db4bbd904fe8840853b589b032c5041bc060b91bcd9c27b914b56581fbc473550eea74b852238c79963b5adf2419f379e9f5db76784048b48e3937f9f3e732bf  /tmp/clusters-bew.html
db4bbd904fe8840853b589b032c5041bc060b91bcd9c27b914b56581fbc473550eea74b852238c79963b5adf2419f379e9f5db76784048b48e3937f9f3e732bf  /tmp/clusters-newer.html
db4bbd904fe8840853b589b032c5041bc060b91bcd9c27b914b56581fbc473550eea74b852238c79963b5adf2419f379e9f5db76784048b48e3937f9f3e732bf  /tmp/clusters-old.html
```

Reviewers: courbet, gchatelet

Reviewed By: courbet

Subscribers: tschuett, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57658

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353025 91177308-0d34-0410-b5e6-96231b3b80d8