Added vm_ivar benchmark for initializing an embedded obj

Merged: https://github.com/ruby/ruby/pull/6030

benchmark/vm_ivar_extended_obj_init.yml

@@ -0,0 +1,14 @@
+prelude: |
+  class C
+    def initialize
+      @a = nil
+      @b = nil
+      @c = nil
+      @d = nil
+      @e = nil
+    end
+  end
+benchmark:
+  vm_ivar_extended_obj_init: |
+    C.new
+loop_count: 30000000