Emitting unreachable for unemittable String instructions can fail due to blocks

[kripken]

Emitting unreachable for unemittable String instructions can fail due to blocks

[kripken]

Another option for now might be to ignore the V8 error, which looks like this:

string.new_wtf16_array[0] expected array of i16, found ref.as_non_null of type (ref none)

but that might be risky to do.

[tlively]

Another option would be to unconditionally emit a cast to the expected type before string.new_wtf16_array, essentially adding our own type annotation. That adds code size, of course, but stringref is EOL so that seems fine to me.