TypeRefiningGUFA: Avoid bottom continuation casts

[kripken]

The pass already checked isContinuation and did not refine a field to
such a type, if we know we need a cast to fix things up (we can't do it
in such cases, as the type can't be cast). However, we did not handle
a cast to the bottom type nullcontref.

Avoid a cast there by emitting a null. That is, before we did

(ref.cast nullcontref
  VALUE
)

and now we do

(block
  (drop (VALUE))
  (ref.null)
)

The null has the right type for where we are writing.

For a non-nullable bottom type, emit an unreachable.

As a drive by, remove

updater.runOnModuleCode(getPassRunner(), &wasm);

That code cannot work in module code anyhow (it emits casts,
blocks, drops, etc.).

[kripken]

Diff on tests is smaller without whitespace.