[WebAssembly] Don't modify preds/succs iterators while erasing from them

Summary:
This caused out-of-bound bugs. Found by
`-DLLVM_ENABLE_EXPENSIVE_CHECKS=ON`.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D52902

llvm-svn: 343814

llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp

@@ -91,12 +91,15 @@ static void EraseBBsAndChildren(const Container &MBBs) {
   SmallVector<MachineBasicBlock *, 8> WL(MBBs.begin(), MBBs.end());
   while (!WL.empty()) {
     MachineBasicBlock *MBB = WL.pop_back_val();
-    for (auto *Pred : MBB->predecessors())
+    SmallVector<MachineBasicBlock *, 4> Preds(MBB->pred_begin(),
+                                              MBB->pred_end());
+    for (auto *Pred : Preds)
       Pred->removeSuccessor(MBB);
-    for (auto *Succ : MBB->successors()) {
-      WL.push_back(Succ);
+    SmallVector<MachineBasicBlock *, 4> Succs(MBB->succ_begin(),
+                                              MBB->succ_end());
+    WL.append(MBB->succ_begin(), MBB->succ_end());
+    for (auto *Succ : Succs)
       MBB->removeSuccessor(Succ);
-    }
     MBB->eraseFromParent();
   }
 }