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[cc2] Add general tree optimizations for jumps and labels

These commit adds:
	- Remove consecutive labels
	- Jump to jump
	- Jump to next instruction

--- a/cc2/optm.c

+++ b/cc2/optm.c

@@ -1,4 +1,6 @@

+#include <stddef.h>

+

 #include "arch.h"

 #include "cc2.h"

@@ -5,14 +7,38 @@

 Node *

 optm(Node *np)

 {

-	Node *dst;

+	Node *p, *dst, *next = np->next;

+	Symbol *sym, *osym;

 	switch (np->op) {

+	case ONOP:

+		if (next && next->op == ONOP) {

+			sym = np->u.sym;

+			osym = next->u.sym;

+			osym->id = sym->id;

+			osym->numid = sym->id;

+			osym->u.stmt = sym->u.stmt;

+			return NULL;

+		}

+		break;

 	case OJMP:

 	case OBRANCH:

-		dst = np->u.sym->u.stmt;

-		if (dst->op == OJMP)

+		for (;;) {

+			dst = np->u.sym->u.stmt;

+			if (dst->op != OJMP)

+				break;

 			np->u.sym = dst->u.sym;

+		}

+		for (p = np->next; p; p = p->next) {

+			if (p == dst)

+				return NULL;

+			if (p->op == ONOP ||

+			    p->op == OBLOOP ||

+			    p->op == OELOOP) {

+				continue;

+			}

+			break;

+		}

 		break;

 	}

 	return np;