Fix determination when slot types for upper executor nodes are fixed.

For many queries the fact that the tuple descriptor from the lower
node was not taken into account when determining whether the type of a
slot is fixed, lead to tuple deforming for such upper nodes not to be
JIT accelerated.

I broke this in 675af5c0.

There is ongoing work to enable writing regression tests for related
behavior (including a patch that would have detected this
regression), by optionally showing such details in EXPLAIN. But as it
seems unlikely that that will be suitable for stable branches, just
merge the fix for now.

While it's fairly close to the 12 release window, the fact that 11
continues to perform JITed tuple deforming in these cases, that
there's still cases where we do so in 12, and the fact that the
performance regression can be sizable, weigh in favor of fixing it
now.

Author: Andres Freund
Discussion: https://postgr.es/m/20190927072053.njf6prdl3vb7y7qb@alap3.anarazel.de
Backpatch: 12-, where 675af5c0 was merged.

src/backend/executor/execExpr.c

@@ -2395,6 +2395,7 @@ ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 		{
 			isfixed = true;
 			tts_ops = parent->innerops;
+			desc = ExecGetResultType(is);
 		}
 		else if (is)
 		{
@@ -2414,6 +2415,7 @@ ExecComputeSlotInfo(ExprState *state, ExprEvalStep *op)
 		{
 			isfixed = true;
 			tts_ops = parent->outerops;
+			desc = ExecGetResultType(os);
 		}
 		else if (os)
 		{