[SLP][NFC] Rename a couple of variables and replace an if-else with an std::min

- Rename `LimitForRegisterSize` to `MaxVFOnly` to make the meaning of the limit less ambiguous
- Rename `OpsWidth` to `ActualVF`, which makes it clear that this is the VF we are using for vectorization.
- Replace the if-else code for the initialization of OpsWidth with an std::min.

Differential Revision: https://reviews.llvm.org/D150241