RFR: 8258961: devirtualize SafePointNode::jvms() and hide out set_jvms()

[Xin Liu]

SafePointNode::jvms() declares virtual but has never been overridden. It seems unnecessary.
The member variable _jvms declares 'JVMState* const', but set_jvms() updates its value anyway.
Geting rid of the const qualifier and hides if out using protected.

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Commit messages:
 - 8258961: devirtualize SafePointNode::jvms() and hide out set_jvms()

Changes: https://git.openjdk.java.net/jdk/pull/1899/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258961
  Stats: 34 lines in 5 files changed: 13 ins; 11 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1899.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1899/head:pull/1899

PR: https://git.openjdk.java.net/jdk/pull/1899