[llvm] [VPlan] Rewrite sinkScalarOperands in preparation to extend (PR #151696)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> It looks like there are some crashes with this patch:

Thanks, there was a thinko:

```diff
@@ -170,10 +170,10 @@ static bool sinkScalarOperands(VPlan &Plan) {
   }

   // Try to sink each replicate or scalar IV steps recipe in the worklist.
-  for (const auto &Item : WorkList) {
+  for (unsigned I = 0; I != WorkList.size(); ++I) {
     VPBasicBlock *SinkTo;
     VPSingleDefRecipe *SinkCandidate;
-    std::tie(SinkTo, SinkCandidate) = Item;
+    std::tie(SinkTo, SinkCandidate) = WorkList[I];
```

... but now there are more crashes. Investigating.

https://github.com/llvm/llvm-project/pull/151696