Fix a bug in the optimer where to optimalization where improperly com…

…bining

This was causing problems on the js and jvm backend.
When optimizing $var += ... into nqp::assign_i($var, $var + ...) $var was
being used twice.
When optimizing $var + ... and turning $var from lexicalref to lexical $var use
in nqp::assign_i($var, ...) was also affected.

src/Perl6/Optimizer.nqp

@@ -2006,7 +2006,9 @@ class Perl6::Optimizer {
               else {
                 return NQPMu;
               }
-              $assignee := $assignee_var := $op[1];
+              $assignee := $op[1];
+              # other optimizations might want to change this independently
+              $assignee_var := nqp::clone($op[1]);
             }
             else {
               $assignop := 'p6store';