Make FlatIO work (as IO) for non-Record Data (#3538)

Change `FlatIO` to work for any `Data` as opposed to just for `Record`.
For non-`Record`, have this behave the same as `IO`.  I.e., there is no
prefix stripping.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>