RFR: 8301047: Clean up type unsafe uses of oop from compiler code

[Erik Österlund]

In an ideal world, when you have an oop, its contents should also be an oop. It happens, however, that we have other forms of pointers sometimes, used with the oop type, in some compiler code. We should clean that up.

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Commit messages:
 - 8301047: Clean up type unsafe uses of oop from compiler code

Changes: https://git.openjdk.org/jdk/pull/12223/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12223&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301047
  Stats: 19 lines in 5 files changed: 1 ins; 1 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/12223.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12223/head:pull/12223

PR: https://git.openjdk.org/jdk/pull/12223