Make transformAExprIn() return a flattened bool expression directly

[cca5507]

Hi,

Now transformAExprIn() returns a bool expression tree. Attach a patch
to make it return a flattened bool expression directly without extra cost.
This can reduce some work of the planner.

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Regards,
ChangAo Chen

v1-0001-Make-transformAExprIn-return-a-flattened-bool-exp.patch
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