RFR: 8266904: Use function pointer typedefs in OopOopIterateDispatch

[Stefan Karlsson]

The way our OopIterateClosures are devirtualized is quite complex. I think we can lower the complexity a tiny bit by using a function pointer typedef.

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Commit messages:
 - 8266904: Use function pointer typedefs in OopOopIterateDispatch

Changes: https://git.openjdk.java.net/jdk/pull/3986/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3986&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266904
  Stats: 12 lines in 1 file changed: 6 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3986.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3986/head:pull/3986

PR: https://git.openjdk.java.net/jdk/pull/3986