Avoid unnecessary snapshot-acquisitions in BuildCachedPlan.

I had copied-and-pasted a claim that we couldn't reach this point when
dealing with utility statements, but that was a leftover from when the
caller was required to supply a plan to start with.  We now will go
through here at least once when handling a utility statement, so it
seems worth a check to see whether a snapshot is actually needed.
(Note that analyze_requires_snapshot is quite a cheap test.)

Per suggestion from Yamamoto Takashi.  I don't think I believe that this
resolves his reported assertion failure; but it's worth changing anyway,
just to save a cycle or two.

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