simplify slot logic in alias analysis

The previous code was buggy.  It would put a stack
pointer on the heap when handling "add $foo, 42".
The new code is more straightforward and hopefully
more correct.  Only temporaries with a "stack"
alias class will have a slot pointer.

all.h

@@ -272,6 +272,7 @@ struct Alias {
 		AEsc = 3, /* stack escaping */
 		ASym = 4,
 		AUnk = 6,
+	#define astack(t) ((t) & 1)
 	} type;
 	Ref base;
 	char label[NString];