RFR: Add flag to prevent multiple deopts on the same nmethod.

[Coleen Phillimore]

I added a flag to prevent the same nmethod from being optimized multiple times.  It might be better to have a new MarkForDeoptimizationStatus enum but wasn't sure how that would interact with the deoptimize_update state.  Maybe that's better.
I also don't know why the sweeper is so slow to get rid of these entries.
This makes ctw_2 test run in an hour on my machine, just like mainline jdk/jdk (machine is slow!) in fastdebug.

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Commit messages:
 - Add flag to prevent multiple deopts on the same nmethod.

Changes: https://git.openjdk.java.net/loom/pull/81/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=81&range=00
  Stats: 13 lines in 4 files changed: 10 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/loom/pull/81.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/81/head:pull/81

PR: https://git.openjdk.java.net/loom/pull/81