[llvm] [LV]Split store-load forward distance analysis from other checks, NFC (PR #121156)

[Florian Hahn via llvm-commits]

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@@ -4107,6 +4112,7 @@ LoopVectorizationCostModel::computeMaxVF(ElementCount UserVF, unsigned UserIC) {
       LLVM_DEBUG(dbgs() << "LV: No tail will remain for any chosen VF.\n");
       return MaxFactors;
     }
+    MaxPowerOf2RuntimeVF.reset();
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fhahn wrote:

But is this only due to the code below added by the patch?

```
  if (MaxPowerOf2RuntimeVF) {
     // Accept MaxFixedVF if we do not have a tail.
     LLVM_DEBUG(dbgs() << "LV: No tail will remain for any chosen VF.\n");
     return MaxFactors;
   }
```

It is not clear to me why `MaxPowerOf2RuntimeVF` set would mean no tail remains, the only place we can guarantee no tail at the moment is the code just above here, which checks against TC?

https://github.com/llvm/llvm-project/pull/121156