This PR improves the performance of `super` calls. While working on some
Rails optimizations jhawthorn discovered that `super` calls were slower
than expected.

The changes here do the following:

1) Adds a check for whether the call frame is not equal to the method
entry iseq. This avoids the `rb_obj_is_kind_of` check on the next line
which is quite slow. If the current call frame is equal to the method
entry we know we can't have an instance eval, etc.
2) Changes `FL_TEST` to `FL_TEST_RAW`. This is safe because we've
already done the check for `T_ICLASS` above.
3) Adds a benchmark for `T_ICLASS` super calls.
4) Note: makes a chage for `method_entry_cref` to use `const`.

On master the benchmarks showed that `super` is 1.76x slower. Our
changes improved the performance so that it is now only 1.36x slower.

Benchmark IPS:

```
Warming up --------------------------------------
               super   244.918k i/100ms
         method call   383.007k i/100ms
Calculating -------------------------------------
               super      2.280M (± 6.7%) i/s -     11.511M in   5.071758s
         method call      3.834M (± 4.9%) i/s -     19.150M in   5.008444s

Comparison:
         method call:  3833648.3 i/s
               super:  2279837.9 i/s - 1.68x  (± 0.00) slower
```

With changes:

```
Warming up --------------------------------------
               super   308.777k i/100ms
         method call   375.051k i/100ms
Calculating -------------------------------------
               super      2.951M (± 5.4%) i/s -     14.821M in   5.039592s
         method call      3.551M (± 4.9%) i/s -     18.002M in   5.081695s

Comparison:
         method call:  3551372.7 i/s
               super:  2950557.9 i/s - 1.20x  (± 0.00) slower
```

Ruby VM benchmarks also showed an improvement:

Existing `vm_super` benchmark`.

```
$ make benchmark ITEM=vm_super

|          |compare-ruby|built-ruby|
|:---------|-----------:|---------:|
|vm_super  |     21.555M|   37.819M|
|          |           -|     1.75x|
```

New `vm_iclass_super` benchmark:

```
$ make benchmark ITEM=vm_iclass_super

|                 |compare-ruby|built-ruby|
|:----------------|-----------:|---------:|
|vm_iclass_super  |      1.669M|    3.683M|
|                 |           -|     2.21x|
```

This is the benchmark script used for the benchmark-ips benchmarks:

```ruby
require "benchmark/ips"

class Foo
  def zuper; end
  def top; end

  last_method = "top"

  ("A".."M").each do |module_name|
    eval <<-EOM
    module #{module_name}
      def zuper; super; end
      def #{module_name.downcase}
        #{last_method}
      end
    end
    prepend #{module_name}
    EOM
    last_method = module_name.downcase
  end
end

foo = Foo.new

Benchmark.ips do |x|
  x.report "super" do
    foo.zuper
  end

  x.report "method call" do
    foo.m
  end

  x.compare!
end
```

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>
Co-authored-by: John Hawthorn <john@hawthorn.email>