Improve code inferring length of bitmap for JITed tuple deforming.

While discussing comment improvements (see next commit) by Justin
Pryzby, Tom complained about a few details of the logic to infer the
length of the NULL bitmap when building the JITed tuple deforming
function. That bitmap allows to avoid checking the tuple header's
natts, a check which often causes a pipeline stall

Improvements:
a) As long as missing columns aren't taken into account, we can
   continue to infer the length of the NULL bitmap from NOT NULL
   columns following it. Previously we stopped at the first missing
   column.  It's unlikely to matter much in practice, but the
   alternative would have been to document why we stop.
b) For robustness reasons it seems better to also check against
   attisdropped - RemoveAttributeById() sets attnotnull to false, but
   an additional check is trivial.
c) Improve related comments

Discussion: https://postgr.es/m/20637.1555957068@sss.pgh.pa.us
Backpatch: -

src/backend/jit/llvm/llvmjit_deform.c

@@ -103,27 +103,27 @@ slot_compile_deform(LLVMJitContext *context, TupleDesc desc,
 	funcname = llvm_expand_funcname(context, "deform");
 
 	/*
-	 * Check which columns do have to exist, so we don't have to check the
-	 * rows natts unnecessarily.
+	 * Check which columns have to exist, so we don't have to check the row's
+	 * natts unnecessarily.
 	 */
 	for (attnum = 0; attnum < desc->natts; attnum++)
 	{
 		Form_pg_attribute att = TupleDescAttr(desc, attnum);
 
 		/*
-		 * If the column is possibly missing, we can't rely on its (or
-		 * subsequent) NOT NULL constraints to indicate minimum attributes in
-		 * the tuple, so stop here.
+		 * If the column is declared NOT NULL then it must be present in every
+		 * tuple, unless there's a "missing" entry that could provide a
+		 * non-NULL value for it. That in turn guarantees that the NULL bitmap
+		 * - if there are any NULLable columns - is at least long enough to
+		 * cover columns up to attnum.
+		 *
+		 * Be paranoid and also check !attisdropped, even though the
+		 * combination of attisdropped && attnotnull combination shouldn't
+		 * exist.
 		 */
-		if (att->atthasmissing)
-			break;
-
-		/*
-		 * Column is NOT NULL and there've been no preceding missing columns,
-		 * it's guaranteed that all columns up to here exist at least in the
-		 * NULL bitmap.
-		 */
-		if (att->attnotnull)
+		if (att->attnotnull &&
+			!att->atthasmissing &&
+			!att->attisdropped)
 			guaranteed_column_number = attnum;
 	}
 
@@ -298,9 +298,12 @@ slot_compile_deform(LLVMJitContext *context, TupleDesc desc,
 	}
 
 	/*
-	 * Check if's guaranteed the all the desired attributes are available in
-	 * tuple. If so, we can start deforming. If not, need to make sure to
-	 * fetch the missing columns.
+	 * Check if it is guaranteed that all the desired attributes are available
+	 * in the tuple (but still possibly NULL), by dint of either the last
+	 * to-be-deformed column being NOT NULL, or subsequent ones not accessed
+	 * here being NOT NULL.  If that's not guaranteed the tuple headers natt's
+	 * has to be checked, and missing attributes potentially have to be
+	 * fetched (using slot_getmissingattrs().
 	 */
 	if ((natts - 1) <= guaranteed_column_number)
 	{