YJIT: implement optimized codegen for `Array#empty?`

[maximecb]

These are fairly common operations on railsbench (and probably liquid as well): see ruby/ruby-bench#123

We defer to send for opt_empty_p:

fn gen_opt_empty_p(
    jit: &mut JITState,
    ctx: &mut Context,
    asm: &mut Assembler,
    ocb: &mut OutlinedCb,
) -> CodegenStatus {
    // Delegate to send, call the method on the recv
    gen_opt_send_without_block(jit, ctx, asm, ocb)
}

And it looks like it should be easy to implement a fast path for both of these methods. We just need to fetch the length and compare against zero. For some embedded strings, we might even be able to know that the string is not empty at compile-time?

[maximecb]

Implemented String#empty? with Alan & Kokubun in: ruby@6bb576f

Now just missing Array#empty?.