[llvm] [LV] Strengthen calls to collectInstsToScalarize (NFC) (PR #130642)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -5393,20 +5393,21 @@ bool LoopVectorizationCostModel::useEmulatedMaskMemRefHack(Instruction *I,
 }
 
 void LoopVectorizationCostModel::collectInstsToScalarize(ElementCount VF) {
-  // If we aren't vectorizing the loop, or if we've already collected the
-  // instructions to scalarize, there's nothing to do. Collection may already
-  // have occurred if we have a user-selected VF and are now computing the
-  // expected cost for interleaving.
-  if (VF.isScalar() || VF.isZero() || InstsToScalarize.contains(VF))
+  assert(VF.isVector() && "Expected VF >= 2");
+
+  // If we've already collected the instructions to scalarize or the predicated
----------------
artagnon wrote:

Actually, the proposed assert is incorrect, and this comment explains why:

```cpp
  // If we've already collected the instructions to scalarize or the predicated
  // BBs after vectorization, there's nothing to do. Collection may already have
  // occurred if we have a user-selected VF and are now computing the expected
  // cost for interleaving.
```

https://github.com/llvm/llvm-project/pull/130642