[llvm] [SandboxVec][DAG] Implement PredIterator (PR #111604)

[Sriraman Tallam via llvm-commits]

================
@@ -145,6 +210,14 @@ class MemDGNode final : public DGNode {
   static bool classof(const DGNode *Other) {
     return Other->SubclassID == DGNodeID::MemDGNode;
   }
+  iterator preds_begin(DependencyGraph &DAG) override {
+    auto OpEndIt = I->op_end();
+    return PredIterator(skipNonInstr(I->op_begin(), OpEndIt), OpEndIt,
+                        MemPreds.begin(), this, DAG);
----------------
tmsri wrote:

Is it time to move MemPreds into MemDGNode?  

https://github.com/llvm/llvm-project/pull/111604