[SLP] Simplify tryToFindDuplicates() (NFC) (#135766)
This NFC aims to simplify the control-flow and interfaces used in tryToFindDuplicates(). The point is to make it easier to understand where decisions for scalar de-duplication are made.
In particular:
- Limit indentation
- Rename some variables to better match their use case
- Always give consistent outputs for VL and ReuseShuffleIndices. This makes it possible to use the same code for building gather TreeEntry everywhere. This also allows to remove the TryToFindDuplicates lambda.
