Gate IncompleteFunctionComponent which only happens in legacy mode

Author: sebmarkbage

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -4037,6 +4037,9 @@ function beginWork(
       );
     }
     case IncompleteFunctionComponent: {
+      if (disableLegacyMode) {
+        break;
+      }
       const Component = workInProgress.type;
       const unresolvedProps = workInProgress.pendingProps;
       const resolvedProps =

packages/react-reconciler/src/ReactFiberCompleteWork.js

@@ -950,10 +950,15 @@ function completeWork(
   // for hydration.
   popTreeContext(workInProgress);
   switch (workInProgress.tag) {
+    case IncompleteFunctionComponent: {
+      if (disableLegacyMode) {
+        break;
+      }
+      // Fallthrough
+    }
     case LazyComponent:
     case SimpleMemoComponent:
     case FunctionComponent:
-    case IncompleteFunctionComponent:
     case ForwardRef:
     case Fragment:
     case Mode:

packages/react-reconciler/src/getComponentNameFromFiber.js

@@ -36,6 +36,7 @@ import {
   SimpleMemoComponent,
   LazyComponent,
   IncompleteClassComponent,
+  IncompleteFunctionComponent,
   DehydratedFragment,
   SuspenseListComponent,
   ScopeComponent,