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

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@266523 138bc75d-0d04-0410-961f-82ee72b054a4