escape: Implement flood phase.

    
    Walks the connection graphs built in the assign phase from the
    function context's sink, propagating the escape level to each
    visited node and uncovering nodes that leak out of their scope
    which implies they must be heap allocated.
    
    Reviewed-on: https://go-review.googlesource.com/18413

From-SVN: r237453

gcc/go/gofrontend/MERGE

-f768153eb2a7a72587c9c0997955cdbbc70322d0
+1f2f2c77c7ec92efa254e07162a8fc0d22a550e7
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.