RFR: 8293170: Improve encoding of the debuginfo nmethod section [v4]

[Boris Ulasevich]

> This is another pull request to replace https://github.com/openjdk/jdk/pull/10025 change which was blocked as not acceptable (see https://github.com/openjdk/jdk/pull/10025#pullrequestreview-1228216330)
> 
> The objections to change #10025 were:
> - specialized algorithm for given data complicates things, makes it hard to learn, test and support
> - algorithm is changed for DebugInfo, and the benefit is only for one type of data
> - statistics of the debug info data can (will) change, breaking the optimization
> 
> The suggestion was:
> - don't change the core algorithm, but add one on top or underneath the existing one, or reuse off-the-shelf zero-reduction schemes such as Cap'n Proto
> 
> With this change I propose a different approach. Instead of bit coding, the sequence of zeros in a data stream is encoded with a special character that normally never appears in Unsinged5 encoding, followed by a byte containing a number of zeros. In this way the updated algorithm is a pure extension of the existing encoding algorithm: data encoded without the zero-reduction trick is unpacked in the same way as before.
> 
> Currently there are several datasets affected by this change: Dependencies info, OopMap info, LineNumber info, Debug info. Only Debug info has a large number of zeros and gets a significant benefit. I experimented with the Cap'n Proto and lz4 algorithms on DebugInfo. The Unsinged5 algorithm has a better compression rate than these.
> 
> DebugInfo data size is reduced by ~20% (actually, 10-30%, depending on the application). Total nmethod size reduction is ~3%.
> 
> Performance impact: Renaisance and DaCapo benchmarks do not show any difference.

Boris Ulasevich has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - whitespace error
 - addressing review comments: super call, clarifying, and adding tests
 - 8293170: Improve encoding of the debuginfo nmethod section

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Changes: https://git.openjdk.org/jdk/pull/12387/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12387&range=03
  Stats: 341 lines in 5 files changed: 337 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/12387.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12387/head:pull/12387

PR: https://git.openjdk.org/jdk/pull/12387