Only set shape id for CCs on attr_set + ivar

Only ivar reader and writer methods should need the shape ID set on the
inline cache.  We shouldn't accidentally overwrite parts of the CC union
when it's not necessary.

vm_callinfo.h

@@ -348,7 +348,10 @@ vm_cc_new(VALUE klass,
         break;
     }
 
-    vm_cc_attr_index_initialize(cc, INVALID_SHAPE_ID);
+    if (cme->def->type == VM_METHOD_TYPE_ATTRSET || cme->def->type == VM_METHOD_TYPE_IVAR) {
+        vm_cc_attr_index_initialize(cc, INVALID_SHAPE_ID);
+    }
+
     RB_DEBUG_COUNTER_INC(cc_new);
     return cc;
 }