start work on comparisons

There are two things I overlooked so far.

1. Binary instructions like cmp that do not have a result
   in registers need the size suffix sometimes, for example
   when comparing a spill location with a constant.

2. The register allocator needs to be adapted to support the
   comparison instruction: it is not possible to compare two
   spill locations without using a register.

lisc/spill.c

@@ -38,7 +38,7 @@ tmpuse(Ref r, int use, int loop, Fn *fn)
 		return;
 	t = &fn->tmp[r.val];
 	t->nuse += use;
-	t->ndef += 1 - use;
+	t->ndef += !use;
 	t->cost += loop;
 }