RFR: 8361076: Add benchmark for ImageReader in preparation for Valhalla changes [v4]

[David Beaumont]

> Initial benchmark to capture at least some comparative measures of ImageReader performance.
> 
> Current results on my laptop:
> 
> Benchmark                                         Mode  Cnt   Score    Error  Units
> NewImageBenchmark.warmCache_CountAllNodes         avgt    5   0.785 ±  0.140  ms/op
> NewImageBenchmark.coldStart_CountOnly               ss    5  34.051 ± 17.662  ms/op
> NewImageBenchmark.coldStart_InitAndCount            ss    5  31.006 ±  9.674  ms/op
> NewImageBenchmark.coldStart_LoadJavacInitClasses    ss    5   3.752 ±  6.873  ms/op
> 
> The upcoming changes to ImageReader should show significant improvements to these results.

David Beaumont has updated the pull request incrementally with one additional commit since the last revision:

  Adding comment about maintainability.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26044/files
  - new: https://git.openjdk.org/jdk/pull/26044/files/089b68e2..965abec0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26044&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26044&range=02-03

  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26044.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26044/head:pull/26044

PR: https://git.openjdk.org/jdk/pull/26044