fix(core): static trees should be cached on options (vuejs#6826)

Author: jkzing

src/core/instance/render-helpers/render-static.js

@@ -11,8 +11,8 @@ export function renderStatic (
 ): VNode | Array<VNode> {
   // static trees can be rendered once and cached on the contructor options
   // so every instance shares the same cached trees
-  const renderFns = this.$options.staticRenderFns
-  const cached = renderFns.cached || (renderFns.cached = [])
+  const options = this.$options
+  const cached = options.cached || (options.cached = [])
   let tree = cached[index]
   // if has already-rendered static tree and not inside v-for,
   // we can reuse the same tree by doing a shallow clone.
@@ -22,7 +22,7 @@ export function renderStatic (
       : cloneVNode(tree)
   }
   // otherwise, render a fresh tree.
-  tree = cached[index] = renderFns[index].call(this._renderProxy, null, this)
+  tree = cached[index] = options.staticRenderFns[index].call(this._renderProxy, null, this)
   markStatic(tree, `__static__${index}`, false)
   return tree
 }