Prevent reading records from different index when merging temp index

[tangenta]

Enhancement

From #45679 (comment):

tidb/ddl/backfilling.go

Line 549 in cf7ae4c

prefix = t.IndexPrefix()

t.IndexPrefix() is the key like t[tableID]_i. If we use t.IndexPrefix().PrefixNext() here, endKey will be t[tableID]_j?

As a result, we will scan the index records from all indexes.