clean the commutativity + fix bug in emit

The commutativity information only makes sense for
arithmetic expressions.  To account for that, I introduced
a new tri-valued boolean type B3.  Memory operations, for
example, will receive an undefined commutativity trit.

The code emitter was buggy when rega emitted instructions
like 'rax = add 1, rax', this is now fixed using the
commutativity information (we rewrite it in 'rax = add
rax, 1').

lisc/spill.c

@@ -355,7 +355,7 @@ spill(Fn *fn)
 			w = (Bits){{0}};
 			if (rtype(i->arg[1]) == RSym
 			&& !req(i->to, R)
-			&& !opdesc[i->op].commut) {
+			&& opdesc[i->op].comm == F) {
 				/* <arch>
 				 *   here we make sure that we
 				 *   will never have to compile