Avoid conditions where control flow is sufficient

This also fixes a type error since resetTextContent can only be called
on Instances.

Author: sebmarkbage

packages/react-reconciler/src/ReactFiberCommitWork.new.js

@@ -1567,44 +1567,36 @@ function commitPlacement(finishedWork: Fiber): void {
   const parentFiber = getHostParentFiber(finishedWork);
 
   // Note: these two variables *must* always be updated together.
-  let parent;
-  let isContainer;
-  const parentStateNode = parentFiber.stateNode;
   switch (parentFiber.tag) {
-    case HostComponent:
-      parent = parentStateNode;
-      isContainer = false;
+    case HostComponent: {
+      const parent: Instance = parentFiber.stateNode;
+      if (parentFiber.flags & ContentReset) {
+        // Reset the text content of the parent before doing any insertions
+        resetTextContent(parent);
+        // Clear ContentReset from the effect tag
+        parentFiber.flags &= ~ContentReset;
+      }
+
+      const before = getHostSibling(finishedWork);
+      // We only have the top Fiber that was inserted but we need to recurse down its
+      // children to find all the terminal nodes.
+      insertOrAppendPlacementNode(finishedWork, before, parent);
       break;
+    }
     case HostRoot:
-      parent = parentStateNode.containerInfo;
-      isContainer = true;
-      break;
-    case HostPortal:
-      parent = parentStateNode.containerInfo;
-      isContainer = true;
+    case HostPortal: {
+      const parent: Container = parentFiber.stateNode.containerInfo;
+      const before = getHostSibling(finishedWork);
+      insertOrAppendPlacementNodeIntoContainer(finishedWork, before, parent);
       break;
+    }
     // eslint-disable-next-line-no-fallthrough
     default:
       throw new Error(
         'Invalid host parent fiber. This error is likely caused by a bug ' +
           'in React. Please file an issue.',
       );
   }
-  if (parentFiber.flags & ContentReset) {
-    // Reset the text content of the parent before doing any insertions
-    resetTextContent(parent);
-    // Clear ContentReset from the effect tag
-    parentFiber.flags &= ~ContentReset;
-  }
-
-  const before = getHostSibling(finishedWork);
-  // We only have the top Fiber that was inserted but we need to recurse down its
-  // children to find all the terminal nodes.
-  if (isContainer) {
-    insertOrAppendPlacementNodeIntoContainer(finishedWork, before, parent);
-  } else {
-    insertOrAppendPlacementNode(finishedWork, before, parent);
-  }
 }
 
 function insertOrAppendPlacementNodeIntoContainer(

packages/react-reconciler/src/ReactFiberCommitWork.old.js

@@ -1567,44 +1567,36 @@ function commitPlacement(finishedWork: Fiber): void {
   const parentFiber = getHostParentFiber(finishedWork);
 
   // Note: these two variables *must* always be updated together.
-  let parent;
-  let isContainer;
-  const parentStateNode = parentFiber.stateNode;
   switch (parentFiber.tag) {
-    case HostComponent:
-      parent = parentStateNode;
-      isContainer = false;
+    case HostComponent: {
+      const parent: Instance = parentFiber.stateNode;
+      if (parentFiber.flags & ContentReset) {
+        // Reset the text content of the parent before doing any insertions
+        resetTextContent(parent);
+        // Clear ContentReset from the effect tag
+        parentFiber.flags &= ~ContentReset;
+      }
+
+      const before = getHostSibling(finishedWork);
+      // We only have the top Fiber that was inserted but we need to recurse down its
+      // children to find all the terminal nodes.
+      insertOrAppendPlacementNode(finishedWork, before, parent);
       break;
+    }
     case HostRoot:
-      parent = parentStateNode.containerInfo;
-      isContainer = true;
-      break;
-    case HostPortal:
-      parent = parentStateNode.containerInfo;
-      isContainer = true;
+    case HostPortal: {
+      const parent: Container = parentFiber.stateNode.containerInfo;
+      const before = getHostSibling(finishedWork);
+      insertOrAppendPlacementNodeIntoContainer(finishedWork, before, parent);
       break;
+    }
     // eslint-disable-next-line-no-fallthrough
     default:
       throw new Error(
         'Invalid host parent fiber. This error is likely caused by a bug ' +
           'in React. Please file an issue.',
       );
   }
-  if (parentFiber.flags & ContentReset) {
-    // Reset the text content of the parent before doing any insertions
-    resetTextContent(parent);
-    // Clear ContentReset from the effect tag
-    parentFiber.flags &= ~ContentReset;
-  }
-
-  const before = getHostSibling(finishedWork);
-  // We only have the top Fiber that was inserted but we need to recurse down its
-  // children to find all the terminal nodes.
-  if (isContainer) {
-    insertOrAppendPlacementNodeIntoContainer(finishedWork, before, parent);
-  } else {
-    insertOrAppendPlacementNode(finishedWork, before, parent);
-  }
 }
 
 function insertOrAppendPlacementNodeIntoContainer(