Fuzzer generates invalid code for table initializers

[stevenfontanella]

I ran --translate-to-fuzz on the following seed file:

(module
  (import "env" "table" (table 1 1 (ref exn)))
)

 bin/wasm-opt -ttf -if seed.wast -all /dev/null -o /dev/null

And hit a fatal error that validation failed, since a ref.as_non_null was generated in a constant expression (the table initializer):

[wasm-validator error in module] unexpected false: table initializer value must be constant, on
...
 (table $timport$0 1 1 (ref exn) (ref.as_non_null
  (ref.null noexn)
 ))

It looks like it's caused by the code described here.