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

[Sriraman Tallam via llvm-commits]

================
@@ -8,10 +8,45 @@
 
 #include "llvm/Transforms/Vectorize/SandboxVectorizer/DependencyGraph.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/SandboxIR/Instruction.h"
 #include "llvm/SandboxIR/Utils.h"
 
 namespace llvm::sandboxir {
 
+PredIterator::value_type PredIterator::operator*() {
+  // If it's a DGNode, or a MemDGNode with an OpIt != end.
+  if (!isa<MemDGNode>(N) || OpIt != OpItE) {
+    assert(OpIt != OpItE && "Can't dereference end iterator!");
+    Value *OpV = *OpIt;
+    return DAG->getNode(cast<Instruction>(OpV));
+  }
+  // It's a MemDGNode with OpIt == end, so we need to use MemIt.
+  assert(MemIt != cast<MemDGNode>(N)->memPreds().end() &&
+         "Cant' dereference end iterator!");
+  return *MemIt;
----------------
tmsri wrote:

Can you simplify this code a bit for just more readability:

```
 if (!isa<MemDGNode>(N)) {
    assert (OpIt != OpItE && .....);
    return DAG->getNode(cast<Instruction>(*OpIt));
 } 
 if (OpIt != OpItE)
  return DAG->getNode(...);
 // It's a ....
```

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