[llvm-exegesis] Separate tool options into three categories.

[llvm-exegesis] Separate tool options into three categories.

Results in much nicer -help output:
```
$ ./bin/llvm-exegesis -help
USAGE: llvm-exegesis [options]

OPTIONS:

Color Options:

  -color                                         - Use colors in output (default=autodetect)

General options:

  -enable-cse-in-irtranslator                    - Should enable CSE in irtranslator
  -enable-cse-in-legalizer                       - Should enable CSE in Legalizer

Generic Options:

  -help                                          - Display available options (-help-hidden for more)
  -help-list                                     - Display list of available options (-help-list-hidden for more)
  -version                                       - Display the version of this program

llvm-exegesis analysis options:

  -analysis-clustering-epsilon=<number>          - dbscan epsilon for benchmark point clustering
  -analysis-clusters-output-file=<string>        -
  -analysis-display-unstable-clusters            - if there is more than one benchmark for an opcode, said benchmarks may end up not being clustered into the same cluster if the measured performance characteristics are different. by default all such opcodes are filtered out. this flag will instead show only such unstable opcodes
  -analysis-inconsistencies-output-file=<string> -
  -analysis-inconsistency-epsilon=<number>       - epsilon for detection of when the cluster is different from the LLVM schedule profile values
  -analysis-numpoints=<uint>                     - minimum number of points in an analysis cluster

llvm-exegesis benchmark options:

  -ignore-invalid-sched-class                    - ignore instructions that do not define a sched class
  -mode=<value>                                  - the mode to run
    =latency                                     -   Instruction Latency
    =inverse_throughput                          -   Instruction Inverse Throughput
    =uops                                        -   Uop Decomposition
    =analysis                                    -   Analysis
  -num-repetitions=<uint>                        - number of time to repeat the asm snippet
  -opcode-index=<int>                            - opcode to measure, by index
  -opcode-name=<string>                          - comma-separated list of opcodes to measure, by name
  -snippets-file=<string>                        - code snippets to measure

llvm-exegesis options:

  -benchmarks-file=<string>                      - File to read (analysis mode) or write (latency/uops/inverse_throughput modes) benchmark results. “-” uses stdin/stdout.
  -mcpu=<string>                                 - cpu name to use for pfm counters, leave empty to autodetect
```

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356364 91177308-0d34-0410-b5e6-96231b3b80d8