Refactor planning of projection steps that don't need a Result plan node.

The original upper-planner-pathification design (commit 3fc6e2d7f5)
assumed that we could always determine during Path formation whether or not
we would need a Result plan node to perform projection of a targetlist.
That turns out not to work very well, though, because createplan.c still
has some responsibilities for choosing the specific target list associated
with sorting/grouping nodes (in particular it might choose to add resjunk
columns for sorting).  We might not ever refactor that --- doing so would
push more work into Path formation, which isn't attractive --- and we
certainly won't do so for 9.6.  So, while create_projection_path and
apply_projection_to_path can tell for sure what will happen if the subpath
is projection-capable, they can't tell for sure when it isn't.  This is at
least a latent bug in apply_projection_to_path, which might think it can
apply a target to a non-projecting node when the node will end up computing
something different.

Also, I'd tied the creation of a ProjectionPath node to whether or not a
Result is needed, but it turns out that we sometimes need a ProjectionPath
node anyway to avoid modifying a possibly-shared subpath node.  Callers had
to use create_projection_path for such cases, and we added code to them
that knew about the potential omission of a Result node and attempted to
adjust the cost estimates for that.  That was uncertainly correct and
definitely ugly/unmaintainable.

To fix, have create_projection_path explicitly check whether a Result
is needed and adjust its cost estimate accordingly, though it creates
a ProjectionPath in either case.  apply_projection_to_path is now mostly
just an optimized version that can avoid creating an extra Path node when
the input is known to not be shared with any other live path.  (There
is one case that create_projection_path doesn't handle, which is pushing
parallel-safe expressions below a Gather node.  We could make it do that
by duplicating the GatherPath, but there seems no need as yet.)

create_projection_plan still has to recheck the tlist-match condition,
which means that if the matching situation does get changed by createplan.c
then we'll have made a slightly incorrect cost estimate.  But there seems
no help for that in the near term, and I doubt it occurs often enough,
let alone would change planning decisions often enough, to be worth
stressing about.

I added a "dummypp" field to ProjectionPath to track whether
create_projection_path thinks a Result is needed.  This is not really
necessary as-committed because create_projection_plan doesn't look at the
flag; but it seems like a good idea to remember what we thought when
forming the cost estimate, if only for debugging purposes.

In passing, get rid of the target_parallel parameter added to
apply_projection_to_path by commit 54f5c5150.  I don't think that's a good
idea because it involves callers in what should be an internal decision,
and opens us up to missing optimization opportunities if callers think they
don't need to provide a valid flag, as most don't.  For the moment, this
just costs us an extra has_parallel_hazard call when planning a Gather.
If that starts to look expensive, I think a better solution would be to
teach PathTarget to carry/cache knowledge of parallel-safety of its
contents.

src/include/nodes/relation.h

@@ -1274,15 +1274,22 @@ typedef struct HashPath
 /*
  * ProjectionPath represents a projection (that is, targetlist computation)
  *
- * This path node represents using a Result plan node to do a projection.
- * It's only needed atop a node that doesn't support projection (such as
- * Sort); otherwise we just jam the new desired PathTarget into the lower
- * path node, and adjust that node's estimated cost accordingly.
+ * Nominally, this path node represents using a Result plan node to do a
+ * projection step.  However, if the input plan node supports projection,
+ * we can just modify its output targetlist to do the required calculations
+ * directly, and not need a Result.  In some places in the planner we can just
+ * jam the desired PathTarget into the input path node (and adjust its cost
+ * accordingly), so we don't need a ProjectionPath.  But in other places
+ * it's necessary to not modify the input path node, so we need a separate
+ * ProjectionPath node, which is marked dummy to indicate that we intend to
+ * assign the work to the input plan node.  The estimated cost for the
+ * ProjectionPath node will account for whether a Result will be used or not.
  */
 typedef struct ProjectionPath
 {
 	Path		path;
 	Path	   *subpath;		/* path representing input source */
+	bool		dummypp;		/* true if no separate Result is needed */
 } ProjectionPath;
 
 /*