Enable SingleQE join with SegmentGeneralWorkers

[avamingli]

For a parallel join, we may benefit if gather SegmentGeneralWorkers to SingleQE.

Gather(SegmentGeneralWorkers) join SingleQE, return join locus: SingleQE. We may win if we are a parallel-aware join, SingleQE is on the inner side that means there is a chance to generate a parallel join under SingleQE. In this case, we have both side parallel and may benefit. See ex 5_P_2_2 in cbdb_parallel.sql
If not parallel-aware, we are not sure for the benefit and a simgle test shows lower performance, ex: parallel scan on replicated table and join with SingleQE which is a non-parallel plan.

SingleQE join Gather(SegmentGeneralWorkers), return join locus: SingleQE. We may win if gather to SingleQE no matter what parallel-aware is. SingleQE is outer side, there could be a parallel plan under it. So we may benefit even without a shared hash table. Let the planner decide.
See ex 2_P_5_2 in cbdb_parallel.sql

The final locus may be elided to Entry if possible.

Authored-by: Zhang Mingli avamingli@gmail.com

fix #ISSUE_Number

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