RFR: Add flag to prevent multiple deopts on the same nmethod. [v4]

[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.  Advice welcome.
> 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.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  CodeCache::mark_all_nmethods_for_deoptimization does try to make the state go backwards.

-------------

Changes:
  - all: https://git.openjdk.java.net/loom/pull/81/files
  - new: https://git.openjdk.java.net/loom/pull/81/files/0d998c61..2865436f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=81&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=81&range=02-03

  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 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