Suppress unnecessary regex subre nodes in a couple more cases.

This extends the changes made in commit cebc1d34, teaching
parseqatom() to generate fewer or cheaper subre nodes in some edge
cases.  The case of interest here is a quantified atom that is "messy"
only because it has greediness opposite to what preceded it (whereas
captures and backrefs are intrinsically messy).  In this case we don't
need an iteration node, since we don't care where the sub-matches of
the quantifier are; and we might also not need a second concatenation
node.  This seems of only marginal real-world use according to my
testing, but I wanted to get it in before wrapping up this series of
regex performance fixes.

Discussion: https://postgr.es/m/1340281.1613018383@sss.pgh.pa.us

src/backend/regex/regcomp.c

@@ -1216,6 +1216,23 @@ parseqatom(struct vars *v,
 		/* rest of branch can be strung starting from atom->end */
 		s2 = atom->end;
 	}
+	else if (!(atom->flags & (CAP | BACKR)))
+	{
+		/*
+		 * If there's no captures nor backrefs in the atom being repeated, we
+		 * don't really care where the submatches of the iteration are, so we
+		 * don't need an iteration node.  Make a plain DFA node instead.
+		 */
+		EMPTYARC(s, atom->begin);	/* empty prefix */
+		repeat(v, atom->begin, atom->end, m, n);
+		f = COMBINE(qprefer, atom->flags);
+		t = subre(v, '=', f, atom->begin, atom->end);
+		NOERR();
+		freesubre(v, atom);
+		*atomp = t;
+		/* rest of branch can be strung starting from t->end */
+		s2 = t->end;
+	}
 	else if (m > 0 && !(atom->flags & BACKR))
 	{
 		/*
@@ -1271,6 +1288,10 @@ parseqatom(struct vars *v,
 		t->flags |= COMBINE(t->flags, t->child->sibling->flags);
 		top->flags |= COMBINE(top->flags, t->flags);
 
+		/* neither t nor top could be directly marked for capture as yet */
+		assert(t->capno == 0);
+		assert(top->capno == 0);
+
 		/*
 		 * At this point both top and t are concatenation (op == '.') subres,
 		 * and we have top->child = prefix of branch, top->child->sibling = t,
@@ -1291,6 +1312,24 @@ parseqatom(struct vars *v,
 			top->child = t->child;
 			freesrnode(v, t);
 		}
+
+		/*
+		 * Otherwise, it's possible that t->child is not messy in itself, but
+		 * we considered it messy because its greediness conflicts with what
+		 * preceded it.  Then it could be that the combination of t->child and
+		 * the rest of the branch is also not messy, in which case we can get
+		 * rid of the child concatenation by merging t->child and the rest of
+		 * the branch into one plain DFA node.
+		 */
+		else if (t->child->op == '=' &&
+				 t->child->sibling->op == '=' &&
+				 !MESSY(UP(t->child->flags | t->child->sibling->flags)))
+		{
+			t->op = '=';
+			t->flags = COMBINE(t->child->flags, t->child->sibling->flags);
+			freesubreandsiblings(v, t->child);
+			t->child = NULL;
+		}
 	}
 	else
 	{