RFR: 8264395: WB_EnqueueInitializerForCompilation fails with "method holder must be initialized" when called for uninitialized class

[Rahul Raghavan]

https://bugs.openjdk.java.net/browse/JDK-8264395

Reported assert failure, during compilation triggered by Whitebox API `enqueueInitializerForCompilation` method,
is fixed by adding support to to bail out if a class not initialized yet.

Confirmed the new `compiler/whitebox/TestEnqueueInitializerForCompilation.java` test passes with the proposed `whitebox.cpp` changes and fails with reported assert without this fix.
No issues with tier tests.

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Commit messages:
 - 8264395: WB_EnqueueInitializerForCompilation fails with "method holder must be initialized" when called for uninitialized class
 - 8264395: WB_EnqueueInitializerForCompilation fails with method holder must be initialized when called for uninitialized class

Changes: https://git.openjdk.java.net/jdk/pull/3757/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3757&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264395
  Stats: 58 lines in 2 files changed: 57 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3757.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3757/head:pull/3757

PR: https://git.openjdk.java.net/jdk/pull/3757