Fix stack-use-after-scope on Windows in Precompute (WebAssembly#6643)

Create a temp var to store the ChildIterator.

Fixes WebAssembly#6639

Author: mtb0x1

src/passes/Precompute.cpp

@@ -829,7 +829,8 @@ struct Precompute
     }
 
     auto* child = stack[index];
-    for (auto** currChild : ChildIterator(stack[index - 1]).children) {
+    auto childIterator = ChildIterator(stack[index - 1]);
+    for (auto** currChild : childIterator.children) {
       if (*currChild == child) {
         return currChild;
       }