RFR: 8319980: [JVMCI] libgraal should reuse Thread instances as C2 does [v2]

[Tom Rodriguez]

> Reuse Threads instances for compiler threads in the same way C1/C2 do.  This has been in use for several months now.  mach5 testing is underway.

Tom Rodriguez has updated the pull request incrementally with one additional commit since the last revision:

  Simplify logic

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16638/files
  - new: https://git.openjdk.org/jdk/pull/16638/files/c9ec6508..8c43c7d6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16638&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16638&range=00-01

  Stats: 12 lines in 1 file changed: 0 ins; 6 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/16638.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16638/head:pull/16638

PR: https://git.openjdk.org/jdk/pull/16638