[IR] Renamed INTERPRET_SCRIPT --> INTERPRET_ROOT to get it into backtraces

* Discovered that MRI test runner had broken because Kernel#caller
  was returning []. Traced this to the fact that backtrace generation
  was skipping INTERPRET_SCRIPT since it was not present in filtered
  list of Interpreter frame types in FrameType.java.

* Perhaps this test case should be added to rubyspecs to verify that
  output is not an empty array.

def foo
  caller
end
p foo