Performance: `pushDerived` should use `IsSingleShard()` check

[arthurschreiber]

Overview of the Issue

pushDerived in go/vt/vtgate/planbuilder/operators/route_planning.go currently uses an explicit check for engine.EqualUnique opcodes, which means that other single shard routing opcodes will not be merged together.

Using isSingleShard() would allow more queries to be merged and pushed down to MySQL.

An example query that wouldn't be merged today would be an outer query targeting a sharded keyspace, containing a subquery that's comprised of a union of two or more queries that fetch data from an unsharded keyspace. In this case, the inner UNION would be mergeable, but won't be merged together today.

Reproduction Steps

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Binary Version

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Operating System and Environment details

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Log Fragments

[aviralgarg05]

I would like to solve this issue