[SLP]Support vectorization of previously vectorized scalars in split nodes
Patch removes the restriction for the revectorization of the previously
vectorized scalars in split nodes, and moves the cost profitability
check to avoid regressions.
Reviewers: hiraditya, RKSimon
Reviewed By: RKSimon
Pull Request: https://github.com/llvm/llvm-project/pull/134286