Check monitors during bytecode verification

Check monitors during bytecode verification

This adds tracking of monitor-enter and monitor-exit instructions
to the bytecode verifier.  The idea is to guarantee that all lock
operations in a method are paired with unlock operations, whether
the method completes normally or abnormally.

Because of an issue in "dx", the code only verifies that the operation
stack has the right size at all times.  We do not yet confirm that
the correct monitor is being unlocked by monitor-exit (the code is
present but ifdefed out).  Also, when monitor verification is enabled,
we do not add the "can throw" path from monitor-exit to the work list,
potentially causing some "dead code" warnings.  (Not coincidentally,
"dead code" checking is now only enabled in libdvm_assert.so.)

Overall increase in bootstrap verification time is about 9%, dropping
to 6% when the new checks are disabled.

The feature is currently disabled by default.  -Xverifyopt:checkmon
enables it.

Bug 2534655

Change-Id: I0eac54ce2623fb1d48cc80889fcdb4fd69de3231