[lworld] RFR: 8275220: [lworld] Various compiler code cleanups and refactoring

[Tobias Hartmann]

This patch contains various code cleanups, refactorings and small improvements that I cherry-picked from the nullable Q-types prototype work.

Thanks,
Tobias

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Commit messages:
 - Removed assert
 - 8275220: [lworld] Various compiler code cleanups and refactoring

Changes: https://git.openjdk.java.net/valhalla/pull/561/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=561&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275220
  Stats: 348 lines in 25 files changed: 73 ins; 183 del; 92 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/561.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/561/head:pull/561

PR: https://git.openjdk.java.net/valhalla/pull/561