Refactor MergeHelper::MergeUntil a bit (#10943)

Refactor MergeHelper::MergeUntil a bit (#10943)

Summary:
The patch untangles some nested ifs in `MergeHelper::MergeUntil`. This will come in handy when extending the compaction logic to support `Merge` for wide-column entities, and also enables us to eliminate some repeated branching on value type and to decrease the scope of some variables.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10943

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D41201946

Pulled By: ltamasi

fbshipit-source-id: 890bd3d4e31cdccadca614489a94686d76485ba9