[llvm] [SLP] Simplify buildTree() (NFC) (PR #138833)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

> LGTM. Could we consider using a different name for `ScalarsVectorizationLegality`? The abbreviation `SVL` might be misleading, as it sounds related to `ArrayRef<Value *> VL`, even though it isn't. This could cause confusion for users.

I chose the names to be similar to the `LoopVectorizer`, where `LoopVectorizationLegality` is often abbreviated as `LVL`. But I agree it be misleading. 

Do you have any suggestion to remove the ambiguity? Maybe I could just name the variable `Legality`, i.e.
```
  ScalarsVectorizationLegality Legality =
      getScalarsVectorizationLegality(VL, Depth, UserTreeIdx);
  const InstructionsState &S = SVL.getInstructionsState();
  if (!Legality.isLegal()) {
    ...
  }
```

https://github.com/llvm/llvm-project/pull/138833