subrepos/binaryen: Don't error out when EH is used for Flatten (#3526…

…) (89b811699b2db0c0e2ad744a1240f6283f8eed86)

We used to error out in Flatten when EH is used because Flatten makes
all block types `none` by setting their return values to locals and
getting them later, but `br_on_exn` by definition pops value from the
value stack at the end of a block so it couldn't be flattened. Now that
we don't have `br_on_exn` we don't need these restriction.