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/test/cup.ssa

@@ -0,0 +1,10 @@
+# counts up
+
+@start
+@loop
+	%n0  =w phi @start -1988, @loop %n1
+	%n1  =w add 1, %n0
+	%cmp =w csle %n1, 1000
+	jez %cmp, @end, @loop
+@end
+	ret