fix(compiler-core): avoid generating useless createVNode helper (#2938)

close #2739

packages/compiler-ssr/src/transforms/ssrTransformComponent.ts

@@ -298,8 +298,17 @@ function subTransform(
   traverseNode(childRoot, childContext)
   // merge helpers/components/directives into parent context
   ;(['helpers', 'components', 'directives'] as const).forEach(key => {
-    childContext[key].forEach((value: any) => {
-      ;(parentContext[key] as any).add(value)
+    childContext[key].forEach((value: any, helperKey: any) => {
+      if (key === 'helpers') {
+        const parentCount = parentContext.helpers.get(helperKey)
+        if (parentCount === undefined) {
+          parentContext.helpers.set(helperKey, value)
+        } else {
+          parentContext.helpers.set(helperKey, value + parentCount)
+        }
+      } else {
+        ;(parentContext[key] as any).add(value)
+      }
     })
   })
   // imports/hoists are not merged because: