Slightly improve performance of hydration. (facebook#15998)

* Slightly improve performance of hydration.

Avoid loading nodeType and data couple times from the same node in a row,
but instead load them only once, which will help engines to run this code
faster, especially during startup of the application. The general approach
is still not ideal, since hydrating this way forces the browser engine
to materialize JavaScript wrapper objects for all DOM nodes, even if they
are not interesting to hydration itself.

* Fix condition for COMMENT_NODEs.

* Improve general code readability

Author: bmeurer

packages/react-dom/src/client/ReactDOMHostConfig.js

@@ -670,44 +670,40 @@ export function registerSuspenseInstanceRetry(
   instance._reactRetry = callback;
 }
 
-export function getNextHydratableSibling(
-  instance: HydratableInstance,
-): null | HydratableInstance {
-  let node = instance.nextSibling;
+function getNextHydratable(node) {
   // Skip non-hydratable nodes.
-  while (
-    node &&
-    node.nodeType !== ELEMENT_NODE &&
-    node.nodeType !== TEXT_NODE &&
-    (!enableSuspenseServerRenderer ||
-      node.nodeType !== COMMENT_NODE ||
-      ((node: any).data !== SUSPENSE_START_DATA &&
-        (node: any).data !== SUSPENSE_PENDING_START_DATA &&
-        (node: any).data !== SUSPENSE_FALLBACK_START_DATA))
-  ) {
-    node = node.nextSibling;
+  for (; node != null; node = node.nextSibling) {
+    const nodeType = node.nodeType;
+    if (nodeType === ELEMENT_NODE || nodeType === TEXT_NODE) {
+      break;
+    }
+    if (enableSuspenseServerRenderer) {
+      if (nodeType === COMMENT_NODE) {
+        break;
+      }
+      const nodeData = (node: any).data;
+      if (
+        nodeData === SUSPENSE_START_DATA ||
+        nodeData === SUSPENSE_FALLBACK_START_DATA ||
+        nodeData === SUSPENSE_PENDING_START_DATA
+      ) {
+        break;
+      }
+    }
   }
   return (node: any);
 }
 
+export function getNextHydratableSibling(
+  instance: HydratableInstance,
+): null | HydratableInstance {
+  return getNextHydratable(instance.nextSibling);
+}
+
 export function getFirstHydratableChild(
   parentInstance: Container | Instance,
 ): null | HydratableInstance {
-  let next = parentInstance.firstChild;
-  // Skip non-hydratable nodes.
-  while (
-    next &&
-    next.nodeType !== ELEMENT_NODE &&
-    next.nodeType !== TEXT_NODE &&
-    (!enableSuspenseServerRenderer ||
-      next.nodeType !== COMMENT_NODE ||
-      ((next: any).data !== SUSPENSE_START_DATA &&
-        (next: any).data !== SUSPENSE_FALLBACK_START_DATA &&
-        (next: any).data !== SUSPENSE_PENDING_START_DATA))
-  ) {
-    next = next.nextSibling;
-  }
-  return (next: any);
+  return getNextHydratable(parentInstance.firstChild);
 }
 
 export function hydrateInstance(