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

[aheejin]

Summary

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

[dschuff]

lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp

94

Could you just copy the Predecessors in the constructor, the way you do with WL?

[dschuff]

Otherwise LGTM, and I verified that it fixes the expensive checks failure.

lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp

94

e.g. Preds(MBB->pred_begin(), MBB->pred_end())

[aheejin]

• Use iterators within constructors / append

lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp

 // Erases the given BBs and all their children from the function. If other BBs
 // have the BB as a successor, the successor relationships will be deleted as
 // well.
 template <typename Container>
 static void EraseBBsAndChildren(const Container &MBBs) {
   SmallVector<MachineBasicBlock *, 8> WL(MBBs.begin(), MBBs.end());
   while (!WL.empty()) {
     MachineBasicBlock *MBB = WL.pop_back_val();
+    SmallVector<MachineBasicBlock *, 4> Preds, Succs;
     for (auto *Pred : MBB->predecessors())
+      Preds.push_back(Pred);
+    for (auto *Pred : Preds)
       Pred->removeSuccessor(MBB);
     for (auto *Succ : MBB->successors()) {
+      Succs.push_back(Succ);
       WL.push_back(Succ);
-      MBB->removeSuccessor(Succ);
     }
+    for (auto *Succ : Succs)
+      MBB->removeSuccessor(Succ);
     MBB->eraseFromParent();
   }
 }
 
 bool WebAssemblyLateEHPrepare::runOnMachineFunction(MachineFunction &MF) {
   if (MF.getTarget().getMCAsmInfo()->getExceptionHandlingType() !=
       ExceptionHandling::Wasm)
     return false;