RFR: 8361076: Add benchmark for ImageReader in preparation for Valhalla changes

[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.

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Commit messages:
 - ImageReader benchmark

Changes: https://git.openjdk.org/jdk/pull/26044/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26044&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361076
  Stats: 1061 lines in 2 files changed: 1061 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