enable tidb_enable_index_merge by default

## Enhancement
Many application query will use sql as below format, although there are both c1/c2 are indexed by separate index.
Without enabling indexMerge, the query is forced full table scan.
I would like to propose to enable `tidb_enable_index_merge` by default.
```
select * from t where c1 = 'abc' or c2 = 'xyz';
index1 on (c1)
index2 on (c2)
```