Request Review: 6902182: Starting with jdwp agent should not incur performance penalty

[Deneau, Tom]

New webrev is at  http://cr.openjdk.java.net/~tdeneau/6902182/webrev.02/

This rev changes

   * two places in the compiler where code for exception throws is
     being JITted (see parse2.cpp, graphKit.cpp).  I was thinking the
     common code in these two should be extracted to one place but I
     wasn't sure whether that belonged in graphKit.cpp or in
     macro.cpp.

   * trace_exception in opto/runtime.cpp

   * exception_handler_for_pc_helper in c1/c1_Runtime1.cpp


Previously these places checked jvmti_can_post_exceptions() which only
looked at whether the jvmti capabilities for exceptions were enabled,
taking a slow path if true.

Now they check a new flag JavaThread::_must_post_exception_events.
This new flag is updated by calling jvmtiExport::must_post_exception_events
whenever the jvmti situation for a thread might have changed.

jvmtiExport::must_post_exception_events uses logic similar to that
used by jvmtiExport::post_exception_throw and returns false if
jvmtiExport::post_exception_throw wouldn't have done anything.

I would appreciate it if someone familiar with the jvmti codepaths
could review this to make sure that the must_post_exception_events
flag is being updated in all the necessary places.  Right now, it gets
updated in
   * JavaThread::set_jvmti_thread_state
   * JvmtiEventControllerPrivate::recompute_enabled