compiler: delay escaping sliced arrays

compiler: delay escaping sliced arrays

    Arrays that are sliced are set to escape in type checking, very
    early in compilation. The escape analysis runs later but cannot
    undo it. This CL changes it to not escape in the early stage.
    Later the escape analysis will make it escape when needed.

    Reviewed-on: https://go-review.googlesource.com/85635

From-SVN: r256403