fix broken optimization for wordvar - value expressions

examples/test.p8

@@ -1,10 +1,14 @@
-main {
-    struct element {
-        word  y
-    }
+%import textio
+%zeropage basicsafe
 
+main {
     sub start() {
-        ^^element zp_element = 20000
-        zp_element.y = cx16.r0L as byte
+        uword @shared z = 100
+        ubyte @shared x = 200
+
+        for x in 15 downto 1 {
+            txt.print_uw(x*$0002-z)     ; TODO fix 6502 optimization bug
+            txt.nl()
+        }
     }
 }