The code that lowers field references can sometimes introduce a double
    pointer indirection in cases where it is not/safe appropriate. For
    example, in
    
            var p **struct { f int }
            p.f = 0
    
    the assignment LHS was being incorrectly lowered to (*(*p)).f.
    Detect this situation and issue an error.
    
    Fixes golang/go#21770
    
    Reviewed-on: https://go-review.googlesource.com/62330

From-SVN: r251918