ART: Fix frontend to allow possibility of multiple code units

This patch addresses a potential problem in frontend which refers
to the CompilationUnit's code unit. However, a MIRGraph may have
multiple code units and thus it makes sense to refer to the current
one being handled.

Change-Id: I69ca7b6056652109c969e7c583fc223e45315c96
Signed-off-by: Razvan A Lupusoru <razvan.a.lupusoru@intel.com>