[llvm] [VPlan] Add VPInstruction to unpack vector values to scalars. (PR #155670)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -3755,6 +3764,48 @@ void VPlanTransforms::materializeBuildVectors(VPlan &Plan) {
           });
     }
   }
+
+  // Create explicit VPInstructions to convert vectors to scalars.
+  for (VPBasicBlock *VPBB :
+       concat<VPBasicBlock *>(VPBBsOutsideLoopRegion, VPBBsInsideLoopRegion)) {
+    for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
+      if (isa<VPReplicateRecipe, VPInstruction, VPScalarIVStepsRecipe>(&R))
+        continue;
+      for (VPValue *Def : R.definedValues()) {
+        if (vputils::isSingleScalar(Def) || vputils::onlyFirstLaneUsed(Def))
+          continue;
+
+        if (VPBB->getParent() != Plan.getVectorLoopRegion())
----------------
artagnon wrote:

```suggestion
        if (VPBB->getParent() != LoopRegion)
```

Can this ever happen? I thought we were doing a shallow traversal?

https://github.com/llvm/llvm-project/pull/155670