RFR: 8303881: Mixed, minor cleanup in jdk.compiler

[Pavel Rappo]

Please review trivial changes mostly to comments.

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Commit messages:
 - Extra fixes
 - Initial commit from 8303755

Changes: https://git.openjdk.org/jdk/pull/12937/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12937&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303881
  Stats: 25 lines in 8 files changed: 0 ins; 3 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/12937.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12937/head:pull/12937

PR: https://git.openjdk.org/jdk/pull/12937