Remove `opt_aref_with` and `opt_aset_with`

When these instructions were introduced it was common to read from a
hash with mutable string literals.  However, these days, I think these
instructions are fairly rare.

I tested this with the lobsters benchmark, and saw no difference in
speed.  In order to be sure, I tracked down every use of this
instruction in the lobsters benchmark, and there were only 4 places
where it was used.

Additionally, this patch fixes a case where "chilled strings" should
emit a warning but they don't.

```ruby
class Foo
  def self.[](x)= x.gsub!(/hello/, "hi")
end

Foo["hello world"]
```

Removing these instructions shows this warning:

```
> ./miniruby -vw test.rb
ruby 3.5.0dev (2025-08-25T21:36:50Z rm-opt_aref_with dca08e286c) +PRISM [arm64-darwin24]
test.rb:2: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
```

[Feature #21553]

0 files changed, 0 insertions, 0 deletions