Fix bug that prohibited assignments with pointer expressions on the LHS

Previously, code like "*(ptr+1) = foo" would claim that the LHS was invalid
for an assignment expression.

Issue #138.

tests_errors/lvalue-3.ispc

@@ -1,4 +1,4 @@
-// Left hand side of assignment statement can't be assigned to
+// Can't assign to type "const uniform int32" on left-hand side of expression
 
 int bar(){ 
     int x;