This CL ports recent enhancements of the escape analysis in the
    gc compiler to gofrontend.
    
    - CL 99335: unnamed receiver should not escape.
    
    - CL 105257: propagate loop depth to field. This prevents it
      from escaping when a field's address is taken inside a loop
      (but not otherwise escape).
    
    - CL 107597: use element type for "indirection" of slice/string.
      This prevents the slice/string from escaping when only the
      element, in case that it is pointerless, flows to outer scope.
    
    Reviewed-on: https://go-review.googlesource.com/120760

From-SVN: r262120