[llvm] [VPlan] Use VPlan predecessors in VPWidenPHIRecipe (NFC). (PR #126388)

[via llvm-commits]

================
@@ -3603,6 +3603,27 @@ void VPReductionPHIRecipe::print(raw_ostream &O, const Twine &Indent,
 }
 #endif
 
+VPBasicBlock *VPWidenPHIRecipe::getIncomingBlock(unsigned I) {
+  VPBasicBlock *Parent = getParent();
+  VPBlockBase *Pred = nullptr;
+  if (Parent->getNumPredecessors() == 0) {
+    auto *Region = Parent->getParent();
+    assert(Region && !Region->isReplicator() && Region->getEntry() == Parent &&
+           "must be in the entry block of a non-replicate region");
+    assert(
+        I < 2 && getNumOperands() == 2 &&
+        "when placed in an entry block, only 2 incoming blocks are available");
+
+    // I ==  0 selects the predecessor of the region, I == 1 selects the region
+    // itself whose exiting block feeds the phi across the backedge.
+    Pred = I == 0 ? Region->getSinglePredecessor() : Region;
+  } else {
+    Pred = Parent->getPredecessors()[I];
+  }
----------------
ayalz wrote:

nit: better start with the 1-liner case first
```suggestion
  if (Parent->getNumPredecessors() > 0) {
    Pred = Parent->getPredecessors()[I];
  } else {
    auto *Region = Parent->getParent();
    assert(Region && !Region->isReplicator() && Region->getEntry() == Parent &&
           "must be in the entry block of a non-replicate region");
    assert(
        I < 2 && getNumOperands() == 2 &&
        "when placed in an entry block, only 2 incoming blocks are available");

    // I ==  0 selects the predecessor of the region, I == 1 selects the region
    // itself whose exiting block feeds the phi across the backedge.
    Pred = I == 0 ? Region->getSinglePredecessor() : Region;
  }
```

https://github.com/llvm/llvm-project/pull/126388