8273956: Add checking for rank values

This change does 3 things.  I could separate them but this has all been tested 
together and most of the change is mechanical.  The first is a simple rename of 
nonleaf => safepoint.  The second change is to add the enum class Rank which 
only allows subtraction from a base rank, and some additional type checking so 
that rank arithmetic doesn't overlap with a different rank.  Ie if you say 
nosafepoint-n, that doesn't overlap with oopstorage.   The third change is to 
remove the _safepoint_check_always/never flag.  That is now intrinsic in the 
ranking, as all nosafepoint ranks are lower than all safepoint ranks.

Future changes could add rank names in the middle of nosafepoint and safepoint, 
like handshake.  As of now, the rank subtractions isn't unmanageable.  There 
are actually not many nested Mutex.

This is the last of the planned subtasks for Mutex ranking cleanup.  If you 
have other ideas or suggestions, please let me know.

Tested with tier1-8 at one point in time and just retested with tier1-6.

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Commit messages:
 - 8274004: Change 'nonleaf' rank name

Changes: https://git.openjdk.java.net/jdk/pull/5845/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5845&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274004
  Stats: 445 lines in 41 files changed: 98 ins; 117 del; 230 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5845.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5845/head:pull/5845

PR: https://git.openjdk.java.net/jdk/pull/5845

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