compiler: check for notinheap struct at each struct field

compiler: check for notinheap struct at each struct field

    When generating write barriers, we were only checking for a notinheap
    struct at the outermost struct.  That mishandled the case of setting a
    pointer to a notinheap struct as a field of another struct that is not
    notinheap.  This caused an invalid write barrier error when building
    the 1.13 version of the runtime.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192279

From-SVN: r275240