If we have a shape cache we should use it

If there is a shape cache, then we should believe the results instead of
doing a linear search for non-existent items

This fixes a case where checking the index of an undefined ivar would
result in an O(n) search. Now we get O(log n).

Benchmark is as follows:

```ruby
N = ARGV[0].to_i

class ManyIVs
  class_eval "def initialize;" +
    N.times.map { "@a#{_1} = #{_1}" }.join("\n") +
    "end"

  def check
    defined?(@not)
  end
end

class Subclass < ManyIVs
  def initialize
    super
    @foo = 123
  end
end

def t
  s = Process.clock_gettime Process::CLOCK_MONOTONIC
  yield
  Process.clock_gettime(Process::CLOCK_MONOTONIC) - s
end

def test
  a = ManyIVs.new
  b = Subclass.new
  t { 200000.times { a.check; b.check } }
end

puts "#{N},#{test}"
```

On the master branch:

```
$ for i in (seq 1 3 32); ./miniruby test.rb $i; end
1,0.015619999991031364
4,0.013061000005109236
7,0.013365999999223277
10,0.015474999992875382
13,0.017674999980954453
16,0.020055999979376793
19,0.02260500000556931
22,0.0254080000158865
25,0.02806599999894388
28,0.031244999991031364
31,0.034568000002764165
```

On this branch:

```
$ for i in (seq 1 3 32); ./miniruby test.rb $i; end
1,0.015848999988520518
4,0.013225000002421439
7,0.013049000001046807
10,0.010697999998228624
13,0.010902000009082258
16,0.011448000004747882
19,0.01151199999731034
22,0.011539999977685511
25,0.01173300002119504
28,0.011900000012246892
31,0.012278999987756833
```

shape.c

@@ -863,7 +863,13 @@ rb_shape_get_iv_index(rb_shape_t *shape, ID id, attr_index_t *value)
     RUBY_ASSERT(rb_shape_id(shape) != OBJ_TOO_COMPLEX_SHAPE_ID);
 
     if (!shape_cache_get_iv_index(shape, id, value)) {
-        return shape_get_iv_index(shape, id, value);
+        // If it wasn't in the ancestor cache, then don't do a linear search
+        if (shape->ancestor_index && shape->next_iv_index >= ANCESTOR_CACHE_THRESHOLD) {
+            return false;
+        }
+        else {
+            return shape_get_iv_index(shape, id, value);
+        }
     }
 
     return true;