Fix case of window function + aggregate + GROUP BY expression.

In commit 1bc16a94 I added a minor
optimization to drop the component variables of a GROUP BY expression from
the target list computed at the aggregation level of a query, if those Vars
weren't referenced elsewhere in the tlist.  However, I overlooked that the
window-function planning code would deconstruct such expressions and thus
need to have access to their component variables.  Fix it to not do that.

While at it, I removed the distinction between volatile and nonvolatile
window partition/order expressions: the code now computes all of them
at the aggregation level.  This saves a relatively expensive check for
volatility, and it's unclear that the resulting plan isn't better anyway.

Per bug #7535 from Louis-David Mitterrand.  Back-patch to 9.2.