[SLP] Use ScopeExit to update Operands/PrevDist on all paths. (NFC) (#83490)

Use ScopeExit to make sure Operands/PrevDist are updated on all paths in
the loop. This makes it easier to ensure they are updated correctly if
new early continues are added.

Split off from https://github.com/llvm/llvm-project/pull/83283

PR: https://github.com/llvm/llvm-project/pull/83490