escape: Analyze multiple result type assertions.

escape: Analyze multiple result type assertions.

    For multi-result type assertions, the object being converted is hidden
    behind unsafe conversions and calls to runtime methods.  This change
    allows the analysis to make edges between the result of the assertion
    and the object being asserted.

    Reviewed-on: https://go-review.googlesource.com/10661

From-SVN: r225751