compiler: add slice initializers to the GC root list

compiler: add slice initializers to the GC root list

    As of https://golang.org/cl/32917 we can put slice initializers in the
    .data section.  The program can still change the values in those
    slices.  That means that if the slice elements can contain pointers,
    we need to register the entire initializer as a GC root.

    This would be straightforward except that we only have a Bexpression
    for the slice initializer, not an Expression.  So introduce a
    Backend_expression type that wraps a Bexpression as an Expression.

    The test case for this is https://golang.org/cl/33790.

    Reviewed-on: https://go-review.googlesource.com/33792

From-SVN: r243129