Add a benchmark for polymorphic ivar setting

This benchmark demonstrates the performance of setting an instance
variable when the type of object is constantly changing.  This benchmark
should give us an idea of the performance of ivar setting in a
polymorphic environment

benchmark/vm_ivar_set_subclass.yml

@@ -0,0 +1,17 @@
+prelude: |
+  class A
+    def initialize
+      @a = nil
+      @b = nil
+      @c = nil
+      @d = nil
+      @e = nil
+    end
+  end
+  class B < A; end
+  class C < A; end
+benchmark:
+  vm_ivar_init: |
+    B.new
+    C.new
+loop_count: 3000000