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

[David Sherwood 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
----------------
david-arm wrote:

Given that calls to `collectUniformsAndScalars` are effectively protected by `if (VF.isScalar() || Uniforms.contains(VF))` in `collectUniformsAndScalars` then this function should only ever be called once per VF. If I understand correctly `Uniforms` will have an entry for every VF regardless of whether or not we found any uniform values. In this case the new if check you added below can just be an assert I think? For example, you could just have

```
  assert(VF.isVector() && "Expected VF >= 2");
  assert(!InstsToScalarize.contains(VF) && !PredicatedBBsAfterVectorization.contains(VF) && "Function unexpectedly called twice!");
```

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