Enable fastpath on invokesuper (#3021)

Fastpath has not been used for invokesuper since it has set vm_call_super_method on every invocation.
Because it seems to be blocked only by refinements, try enabling fastpath on invokesuper when cme is not for refinements.

While this patch itself should be helpful for VM performance, a part of this patch's motivation is to unblock inlining invokesuper on JIT. 

$ benchmark-driver -v --rbenv 'before;after' benchmark/vm2_super.yml --repeat-count=4
before: ruby 2.8.0dev (2020-04-11T15:19:58Z master a01bda59) [x86_64-linux]
after: ruby 2.8.0dev (2020-04-12T02:00:08Z invokesuper-fastpath c171984ee3) [x86_64-linux]
Calculating -------------------------------------
                         before       after
           vm2_super    20.031M     32.860M i/s -      6.000M times in 0.299534s 0.182593s

Comparison:
                        vm2_super
               after:  32859885.2 i/s
              before:  20031097.3 i/s - 1.64x  slower

vm_insnhelper.c

@@ -3032,7 +3032,7 @@ vm_call_method_each_type(rb_execution_context_t *ec, rb_control_frame_t *cfp, st
 
       case VM_METHOD_TYPE_REFINED:
         // CC_SET_FASTPATH(cc, vm_call_refined, TRUE);
-        // should not set FASTPATH because vm_call_refined check cc->call.
+        // should not set FASTPATH since vm_call_refined assumes cc->call is vm_call_super_method on invokesuper.
         return vm_call_refined(ec, cfp, calling, cd);
     }
 
@@ -3222,7 +3222,8 @@ vm_search_super_method(const rb_control_frame_t *reg_cfp, struct rb_call_data *c
             const struct rb_callcache *cc = vm_cc_new(klass, cme, vm_call_super_method);
             RB_OBJ_WRITE(reg_cfp->iseq, &cd->cc, cc);
         }
-        else {
+        else if (cached_cme->def->type == VM_METHOD_TYPE_REFINED) {
+            // vm_call_refined (search_refined_method) assumes cc->call is vm_call_super_method on invokesuper.
             vm_cc_call_set(cd->cc, vm_call_super_method);
         }
     }