RFR: 8299079: Better interface nmethod oop accesses

[Erik Österlund]

Today when oops are read from nmethods, we use the same interfacing that we would use to read any other IN_NATIVE oops. That isn't enough for generational ZGC. The nmethod oops are rather different from other IN_NATIVE oops in that they are encoded in the machine code as direct pointers. Normally, IN_NATIVE oops with generational ZGC are not direct pointers, but are colored pointered that can be converted to direct pointers by going through a load barrier. For nmethod oops, that conversion requires external knowledge from the surrounding nmethod. I propose a new NMethodAccess to interface better with the GC.

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Commit messages:
 - 8299079: Better interface nmethod oop accesses

Changes: https://git.openjdk.org/jdk/pull/11738/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11738&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299079
  Stats: 24 lines in 3 files changed: 10 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/11738.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11738/head:pull/11738

PR: https://git.openjdk.org/jdk/pull/11738