compiler: guard against implicit double pointer indirection

compiler: guard against implicit double pointer indirection

    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