mx: add check for compliance level of a distribution

--HG--
extra : amend_source : 0e225565d0150c6698edca4fc6cae941d6334a75
fixed race involving NodeRefIterator.Empty
re-enable use of generated nodes by default
ensure NodeRefIterator.Empty.hasNext() returns false
mx: do not clean distributions if the --no-java option is given to the clean command
removed Node.recordsUsages()
denote certain applications of DeadCodeEliminationPhase as optional and only run them if -G:-ReduceDCE
Truffle: byte[] sources.
Merge.