The `newhash` instruction was checking if `num` is greater than 0, but
so is [`rb_hash_new_with_size`](https://github.com/ruby/ruby/blob/82e2443d8b1e3edd2607c78dddf5aac79a13492d/hash.c#L1564)
as well as [`rb_hash_bulk_insert`](https://github.com/ruby/ruby/blob/82e2443d8b1e3edd2607c78dddf5aac79a13492d/hash.c#L4764).

If we know the size is 0 in the instruction, we can just directly call
`rb_hash_new` and only check the size once.  Unfortunately, when num is
greater than 0, it's still checked 3 times.