Make sure return values are assigned an affinity.

When an EBB argument value is used only as a return value, it still
needs to be given a register affinity. Otherwise it would appear as a
ghost value with no affinity.

Do the same to call arguments.

filetests/regalloc/basic.cton

@@ -40,3 +40,26 @@ ebb0(v1: i32, v2: i32):
 ; nextln: return $v2, $v1
         return v2, v1
 }
+
+; Return an EBB argument.
+function %retebb(i32, i32) -> i32 {
+ebb0(v1: i32, v2: i32):
+    brnz v1, ebb1(v1)
+    jump ebb1(v2)
+
+ebb1(v10: i32):
+    return v10
+}
+
+; Pass an EBB argument as a function argument.
+function %callebb(i32, i32) -> i32 {
+    fn0 = function %foo(i32) -> i32
+
+ebb0(v1: i32, v2: i32):
+    brnz v1, ebb1(v1)
+    jump ebb1(v2)
+
+ebb1(v10: i32):
+    v11 = call fn0(v10)
+    return v11
+}