RFR: Loom cleanups 7

[Stefan Karlsson]

More cleanups and fixes

-------------

Commit messages:
 - Unify nmethod following and add comments
 - Fix assert (
 - Fix javaClasses object names
 - Don't cast to oop in functions used from StackChunkAllocator
 - Fix 32-bit exclusion of derived oop high-order bit tagging support

Changes: https://git.openjdk.java.net/loom/pull/141/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=141&range=00
  Stats: 161 lines in 12 files changed: 52 ins; 2 del; 107 mod
  Patch: https://git.openjdk.java.net/loom/pull/141.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/141/head:pull/141

PR: https://git.openjdk.java.net/loom/pull/141