Teach create_projection_plan to omit projection where possible.

Teach create_projection_plan to omit projection where possible.

We sometimes insert a ProjectionPath into a plan tree when projection
is not strictly required. The existing code already arranges to avoid
emitting a Result node when the ProjectionPath's subpath can perform
the projection itself, but previously it didn't consider the
possibility that the parent node might not actually require the
projection to be performed at all.

Skipping projection when it's not required can not only avoid Result
nodes that aren't needed, but also avoid losing the "physical tlist"
optimization unneccessarily.

Patch by me, reviewed by Amit Kapila.

Discussion: http://postgr.es/m/CA+TgmoakT5gmahbPWGqrR2nAdFOMAOnOXYoWHRdVfGWs34t6_A@mail.gmail.com