Remove useless "retry memory" logic within regex engine.

Apparently some primordial version of Spencer's engine needed cdissect()
and child functions to be able to continue matching from a previous
position when re-called.  That is dead code, though, since trivial
inspection shows that cdissect can never be entered without having
previously done zapmem which resets the relevant retry counter.  I have
also verified experimentally that no case in the Tcl regression tests
reaches cdissect with a nonzero retry value.  Accordingly, remove that
logic.  This doesn't really save any noticeable number of cycles in itself,
but it is one step towards making dissect() and cdissect() equivalent,
which will allow removing hundreds of lines of near-duplicated code.

Since struct subre's "retry" field is no longer particularly related to
any kind of retry, rename it to "id".  As of this commit it's only used
for identifying a subre node in debug printouts, so you might think we
should get rid of the field entirely; but I have a plan for another use.

src/backend/regex/regcomp.c

@@ -1623,7 +1623,7 @@ subre(struct vars * v,
 
 	ret->op = op;
 	ret->flags = flags;
-	ret->retry = 0;
+	ret->id = 0;				/* will be assigned later */
 	ret->subno = 0;
 	ret->min = ret->max = 1;
 	ret->left = NULL;
@@ -1693,7 +1693,7 @@ optst(struct vars * v,
 }
 
 /*
- * numst - number tree nodes (assigning retry indexes)
+ * numst - number tree nodes (assigning "id" indexes)
  */
 static int						/* next number */
 numst(struct subre * t,
@@ -1704,7 +1704,7 @@ numst(struct subre * t,
 	assert(t != NULL);
 
 	i = start;
-	t->retry = (short) i++;
+	t->id = (short) i++;
 	if (t->left != NULL)
 		i = numst(t->left, i);
 	if (t->right != NULL)
@@ -1999,11 +1999,11 @@ stid(struct subre * t,
 	 char *buf,
 	 size_t bufsize)
 {
-	/* big enough for hex int or decimal t->retry? */
-	if (bufsize < sizeof(void *) * 2 + 3 || bufsize < sizeof(t->retry) * 3 + 1)
+	/* big enough for hex int or decimal t->id? */
+	if (bufsize < sizeof(void *) * 2 + 3 || bufsize < sizeof(t->id) * 3 + 1)
 		return "unable";
-	if (t->retry != 0)
-		sprintf(buf, "%d", t->retry);
+	if (t->id != 0)
+		sprintf(buf, "%d", t->id);
 	else
 		sprintf(buf, "%p", t);
 	return buf;

src/include/regex/regguts.h

@@ -409,7 +409,7 @@ struct subre
 #define  PREF(f) ((f)&LOCAL)
 #define  PREF2(f1, f2)	 ((PREF(f1) != 0) ? PREF(f1) : PREF(f2))
 #define  COMBINE(f1, f2) (UP((f1)|(f2)) | PREF2(f1, f2))
-	short		retry;			/* index into retry memory */
+	short		id;				/* ID of subre (1..ntree) */
 	int			subno;			/* subexpression number (for 'b' and '(') */
 	short		min;			/* min repetitions for iteration or backref */
 	short		max;			/* max repetitions for iteration or backref */