Fix the behavior of variable symbols in regard to cheap locals. Previously

every assignment to a variable symbol opened the same scope for cheap locals.
So when redefining a variable symbol, an old cheap local scope was reopened
which was unexpected and confusing. The change fixes this so that only the
first definition of a variable symbol opens a new scope for cheap locals, but
redefinitions of the same symbol do not.

test/asm/err/bug505.s

@@ -0,0 +1,28 @@
+; Test for #525 taken from the issue
+; Redefining a variable symbol "reopens" the old name space for cheap locals
+; Behavior should be: First definition of a variable symbol opens a new 
+; scope for cheap locals, redefinitions of the same symbols do not.
+
+;this starts a new scope for cheap local lables
+SomeSymbol .set 4
+
+        jmp @CheapLocal1
+
+@CheapLocal0:
+
+        .byte $8b
+
+CheapLocalScopeBreaker0:
+
+CheapLocalScopeBreaker1:
+
+CheapLocalScopeBreaker2:
+
+CheapLocalScopeBreaker3:
+
+;this continues the same cheap scope as before, regardless of the many global labels in between
+SomeSymbol .set 5
+
+@CheapLocal1:
+
+        lda @CheapLocal0