compiler: make Slice_construction_expression::do_flatten idempotent

compiler: make Slice_construction_expression::do_flatten idempotent

    Because of the way we handle call expressions with multiple results,
    it's possible for expressions to be flattened more than once.  In the
    case of Slice_construction_expression, allocating the slice storage
    multiple times caused a compiler crash as one of the
    Temporary_statement's wound up not getting a backend expression.

    Test case is https://golang.org/cl/34020.

    Reviewed-on: https://go-review.googlesource.com/34026

From-SVN: r243442