compiler: add explicit convert in Type_guard_expression::do_get_backend

compiler: add explicit convert in Type_guard_expression::do_get_backend

    The current recipe in Type_guard_expression for creating a Bexpression
    performs a type conversion at the Type level, but doesn't invoke
    Backend::convert_expression to capture the conversion for the back
    end. Add code to create a BE type conversion operation.

    Reviewed-on: https://go-review.googlesource.com/50373

From-SVN: r250400