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Convert additions to increments

When the immediate value is small is better to use increments instead
of an addition.

--- a/cc2/cgen.c

+++ b/cc2/cgen.c

@@ -243,6 +243,7 @@

 add(Node *np)

 {

 	Node *lp = np->left, *rp = np->right;

+	uint8_t i;

 	if (rp->op == REG || lp->op == CONST) {

 		conmute(np);

@@ -300,7 +301,17 @@

 			abort();

 		}

 	add_A:

+		if (rp->op == CONST) {

+			if  ((i = rp->sym->u.imm) == 0)

+				goto update_A;

+			 if (i < 4) {

+				while (i--)

+					code(INC, lp, NULL);

+				goto update_A;

+			}

+		}

 		code(ADD, lp, rp);

+	update_A:

 		np->op = REG;

 		np->reg = A;

 		break;