escape: Implement assign phase.

escape: Implement assign phase.

    Implementation of the assign/connect phase.
    Statements containing expressions which alias local, parameter,
    and global objects are analyzed and the alias relationship between
    objects containing pointers are captured in a connection graph to
    summarize the assignments within a function.

    Reviewed-on: https://go-review.googlesource.com/18302

From-SVN: r237424