[SLP] Exit early if MaxVF < MinVF (NFCI). (#83283)

Exit early if MaxVF < MinVF. In that case, the loop body below will
never get entered. Note that this adjusts the condition from MaxVF <=
MinVF. If MaxVF == MinVF, vectorization may still be feasible (and the
loop below gets entered).

PR: https://github.com/llvm/llvm-project/pull/83283