compiler: change Expression export to use Export_function_body

compiler: change Expression export to use Export_function_body

    In preparation for writing expressions to inline function bodies,
    change the existing expression export code to use Export_function_body
    rather than Export.  Adjust existing expression exporters accordingly.

    This is a refactoring that doesn't affect compiler output.

    Reviewed-on: https://go-review.googlesource.com/c/150063

From-SVN: r266523