escape: Implement the discovery phase.

escape: Implement the discovery phase.

    Implements the discovery phase according the SCC algorithm used in
    gc/esc.go.  Traverse all functions to find components that are either
    trivial or recursive.  The discovered function groups will be analyzed
    from the bottom-up to allow for an inter-procedural analysis.

    Reviewed-on: https://go-review.googlesource.com/18221

From-SVN: r236224