Add mterp checks after monitor entry/exit opcodes.

Check whether we are allowed to continue running in mterp.

This tries to enforce that we bail out of mterp after suspend point
if needed (in particular, if debugger event switches interpreters).

Test: ./art/test.py -b -r --interpreter
Change-Id: If2d55e5ece22831ef864c181b7d6d5c70ca50e5d