RFR: 8258961: move some fields of SafePointNode from public to protected [v5]

[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.

Xin Liu has updated the pull request incrementally with one additional commit since the last revision:

  8258961: move some fields of SafePointNode from public to protected
  
  revert code change to devirtualize Node::jvms().
  There are 3 nodes override jvms() --  SafePointNode, MachSafePointNode and
  MachHalt. It does not make sense to devirtualize it because we have to do
  non-trivial dispatch based on types in Node::jvms(). It is not worth it.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1899/files
  - new: https://git.openjdk.java.net/jdk/pull/1899/files/44d7b8e7..cdac2d76

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1899&range=03-04

  Stats: 15 lines in 3 files changed: 0 ins; 9 del; 6 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