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[cc2] Remove statements in apply()

A usage case of apply() may be optimization, and we need some way
of removing unwanted statments, and we can use the return value
of the parameter function to mark what statements must be deleted.

--- a/cc2/node.c

+++ b/cc2/node.c

@@ -9,6 +9,7 @@

 #define NNODES   32

+Node *curstmt;

 Symbol *curfun;

 Type rtype;

@@ -18,7 +19,7 @@

 };

 static struct arena *arena;

-static Node *freep, *stmtp;

+static Node *freep;

 static int inhome;

 Node *

@@ -50,14 +51,38 @@

 	if (!curfun->u.stmt)

 		curfun->u.stmt = np;

 	else

-		stmtp->next = np;

+		curstmt->next = np;

 	np->next = NULL;

-	np->prev = stmtp;

-	stmtp = np;

+	np->prev = curstmt;

+	curstmt = np;

 	return np;

 }

+Node *

+delstmt(void)

+{

+	Node *next, *prev;

+

+	next = curstmt->next;

+	prev = curstmt->prev;

+	if (next)

+		next->prev = prev;

+	if (prev)

+		prev->next = next;

+	else

+		curfun->u.stmt = next;

+	deltree(curstmt);

+

+	return curstmt = next;

+}

+

+Node *

+nextstmt(void)

+{

+	return curstmt = curstmt->next;

+}

+

 void

 delnode(Node *np)

 {

@@ -87,17 +112,15 @@

 	}

 	arena = NULL;

 	freep = NULL;

-	stmtp = NULL;

+	curstmt = NULL;

 }

 void

 apply(Node *(*fun)(Node *))

 {

-	Node *np;

-

 	if (!curfun)

 		return;

-

-	for (np = curfun->u.stmt; np; np = np->next)

-		(*fun)(np);

+	curstmt = curfun->u.stmt;

+	while (curstmt)

+		(*fun)(curstmt) ? nextstmt() : delstmt();

 }