[SLP]Skip gathers with erased scalars in the cast context
The cast context of the reduction root is requested after the tree is
vectorized, when the scalars of the gathered root with the vectorized
subvector are already erased and have no operands, causes the crash.
Fixes https://github.com/llvm/llvm-project/pull/224919#issuecomment-5944875384
Reviewers:
Pull Request: https://github.com/llvm/llvm-project/pull/228395
