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[cc2-qbe] Simplify abbrev()

We do not need to allocate memory in abbrev, because we can use a
temporary variable. This code came from the old qbe backend, but
it can be done better with the new code.

--- a/cc2/arch/qbe/cgen.c

+++ b/cc2/arch/qbe/cgen.c

@@ -259,19 +259,12 @@

 static Node *

 abbrev(Node *np, Node *ret)

 {

-	Node *tmp;

+	Node aux;

-	if (np->u.subop == 0)

-		return np->right;

-

-	tmp = newnode(np->u.subop);

-	tmp->type = np->type;

-	tmp->right = np->right;

-	tmp->left = np->left;

-	rhs(tmp, ret);

-	deltree(tmp);

-

-	return ret;

+	tmpnode(&aux, &np->type);

+	aux.right = np->right;

+	aux.left = np->left;

+	return rhs(&aux, ret);

 }

 static Node *

@@ -499,7 +492,9 @@

 	case OCAST:

 		return cast(tp, rhs(l, &aux1), ret);

 	case OASSIG:

-		r = abbrev(np, &aux1);

+		/* TODO: see what is the more difficult */

+		if (np->u.subop != 0)

+			r = abbrev(np, &aux1);

 		lhs(l, &aux2);

 		rhs(r, ret);

 		return assign(&aux2, ret);