RFR: 8340141: C1: rework ciMethod::equals following 8338471

[Dean Long]

This PR changes ciMethod::equals() to a special-purpose debug helper method for the one place in C1 that uses it in an assert.  The reason why making it general purpose is difficult is because JVMTI can add and delete methods.   See the bug report and JDK-8338471 for more details.  I'm open to suggestions for a better name than equals_ignore_version().

An alternative approach, which I think may actually be better, would be to check for old methods first, and bail out if we see any.  Then we can change the assert back to how it was originally, using ==.

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Commit messages:
 - rename and restrict usage

Changes: https://git.openjdk.org/jdk/pull/21148/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21148&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340141
  Stats: 19 lines in 3 files changed: 15 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21148.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21148/head:pull/21148

PR: https://git.openjdk.org/jdk/pull/21148