Load shadow frame's this object only upon instrumentation.

We used to load the shadow frame's this object when entering the interpreter
and push it into thread's SIRT to make it visible to garbage collector. But
it is only used by instrumentation listeners.

We now move this load at each place an instrumentation listener is called. This
avoids loading it when instrumentation is disabled. This also allows to remove
the SIRT reference and the push/pop management it implies when entering/exiting
the interpreter.

The performance improvement is 6% in average on our benchmarks suite.

This CL also makes method Instrumentation::ExceptionCaughtEvent const so we can
use a "const instrumentation::Instrumentation*" object in interpreter.

Change-Id: I2caccba9a906f244c8057b24031250f9824cc711