RFR: 8287281: adjust guarantee in Handshake::execute for the case of target thread being current [v6]

[Johan Sjölén]

> Please review this PR for fixing JDK-8287281.
> 
> If a thread is handshake safe we immediately execute the closure, instead of going through the regular Handshake process. 
> 
> Finally:  Should `VirtualThreadGetThreadClosure` and its `do_thread()` body be inlined instead? We can do this in this PR, imho, but I'm hoping to get some input on this.
> 
> 
> Passes tier1. Running tier2-5.

Johan Sjölén has updated the pull request incrementally with one additional commit since the last revision:

  Move assert up and remove other assert, remove unused var

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8992/files
  - new: https://git.openjdk.java.net/jdk/pull/8992/files/c476d300..f2646e04

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8992&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8992&range=04-05

  Stats: 4 lines in 1 file changed: 1 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8992.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8992/head:pull/8992

PR: https://git.openjdk.java.net/jdk/pull/8992