Wasm: Ensure result of `ref.is_null` is I32
Wasm: Ensure result of `ref.is_null` is I32

The (r32|r64).is_null instruction yields a boolean type, so we must
convert a Wasm `ref.is_null` to an integer so we don't get verifier
errors.

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne