RFR: 8336466: C2: Parser incorrectly/unnecessarily checks for clinits

[Aleksey Shipilev]

As shown in [JDK-8336103](https://bugs.openjdk.org/browse/JDK-8336103), C2 Parser code uses `is_initializer` in the code where we expect to only check for constructors. The fix nominally changes the behavior for C2. I do not believe this is something readily observable, but need to be more explicit.

Additional testing:
 - [x]  Linux AArch64 server fastdebug, all (includes Fuzzer and CTW tests), passed as part of JDK-8336103 testing

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Commit messages:
 - Missed the spot
 - Fix

Changes: https://git.openjdk.org/jdk/pull/20191/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20191&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336466
  Stats: 5 lines in 2 files changed: 0 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20191.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20191/head:pull/20191

PR: https://git.openjdk.org/jdk/pull/20191