Implement opt_aref_with

[maximecb]

It looks like the code generated by erubi makes use of opt_aref_with, and on the benchmark recently added by @noahgibbs on speed.yjit.org, this causes us to exit early, and only get 5% coverage. This instruction seems to be used to index hashes with string literals. It's not super common, but it also shouldn't be very difficult to implement, so may be nice to have.

Top exits on the erubi bechmark:

Top-20 most frequent exit ops (100.0% of exits):
    opt_aref_with:   17263742 (100.0)
      invokeblock:         26 (0.0)
      adjuststack:          0 (0.0)
      anytostring:          0 (0.0)
         branchif:          0 (0.0)
        branchnil:          0 (0.0)

[HParker]

I looked into this with @jhawthorn and the main complexity is that when hashes compare by identity the string needs to be rb_str_resurrect which is not impossible to do in generated code, but would be complicated. https://github.com/ruby/ruby/blob/master/insns.def#L1350-L1357

Another thing to note is Ruby won't generate opt_aref_with when frozen_string_literal is true

for example:

This generates opt_aref_with:

def foo
  h = { "hi" => "ok" }
  h["hi"]
end

This does not:

# frozen_string_literal: true
def foo
  h = { "hi" => "ok" }
  h["hi"]
end

So if this change is merged: Shopify/yjit-bench#63
it won't generate opt_aref_with anymore which maybe makes optimizing this instruction less important.

[maximecb]

Roger that @HParker. We'll close this issue for now. Thanks for taking the time to investigate it 🙏