[llvm] [VPlan] Fix broadcasted values using loop region during execution (PR #142594)

[David Sherwood via llvm-commits]

================
@@ -290,18 +290,22 @@ Value *VPTransformState::get(const VPValue *Def, bool NeedsScalar) {
     return Data.VPV2Vector[Def];
 
   auto GetBroadcastInstrs = [this, Def](Value *V) {
+    VPBasicBlock *PreheaderVPBB = nullptr;
+    if (auto *Header = vputils::getFirstLoopHeader(*Plan, VPDT))
----------------
david-arm wrote:

Looking at the definition of `getFirstLoopHeader`:

```
VPBasicBlock *vputils::getFirstLoopHeader(VPlan &Plan, VPDominatorTree &VPDT) {
  auto DepthFirst = vp_depth_first_shallow(Plan.getEntry());
  auto I = find_if(DepthFirst, [&VPDT](VPBlockBase *VPB) {
    return VPBlockUtils::isHeader(VPB, VPDT);
  });
  return I == DepthFirst.end() ? nullptr : cast<VPBasicBlock>(*I);
}
```

and `isHeader`:

```
bool VPBlockUtils::isHeader(const VPBlockBase *VPB,
                            const VPDominatorTree &VPDT) {
  auto *VPBB = dyn_cast<VPBasicBlock>(VPB);
  if (!VPBB)
    return false;

  // If VPBB is in a region R, VPBB is a loop header if R is a loop region with
  // VPBB as its entry, i.e., free of predecessors.
  if (auto *R = VPBB->getParent())
    return !R->isReplicator() && VPBB->getNumPredecessors() == 0;

  // A header dominates its second predecessor (the latch), with the other
  // predecessor being the preheader
  return VPB->getPredecessors().size() == 2 &&
         VPDT.dominates(VPB, VPB->getPredecessors()[1]);
}
```

It suggests a case where `getFirstLoopHeader` returns a `Header` without any predecessors:

```
  // If VPBB is in a region R, VPBB is a loop header if R is a loop region with
  // VPBB as its entry, i.e., free of predecessors.
```

Does this mean the code on the line below is unsafe because there are no predecessors?

https://github.com/llvm/llvm-project/pull/142594