compiler: set captured variable address to nonescape until further flooded

compiler: set captured variable address to nonescape until further flooded

    The escape analysis models closures by flowing captured variable
    address to the closure node. However, the escape state for the
    address expressions remained unset as ESCAPE_UNKNOWN. This
    caused later passes to conclude that the address escapes. Fix this by
    setting its escape state to ESCAPE_NONE first. If it escapes
    (because the closure escapes), the flood phase will set its
    escape state properly.

    Reviewed-on: https://go-review.googlesource.com/86240

From-SVN: r256411