RFR: 8133873: Simplify {Register,Unregister}NMethodOopClosure

[Albert Mingkun Yang]

Simple change of excluding compressed oops in nmethod closures.

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Commit messages:
 - reg_nmethod

Changes: https://git.openjdk.java.net/jdk/pull/4632/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4632&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8133873
  Stats: 32 lines in 1 file changed: 8 ins; 18 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4632.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4632/head:pull/4632

PR: https://git.openjdk.java.net/jdk/pull/4632