[js] Disable constant folding in the optimizer as it's causing trouble

A .Slip seems to be called when doing folding in the setting.
Due to to a bug with BEGIN time multimethods (also shared on the moarvm) backend it's causing looping.

src/Perl6/Optimizer.nqp

@@ -1418,6 +1418,11 @@ class Perl6::Optimizer {
                     }
                 }
 
+
+#?if js
+                return $op; # Avoid looping on Slip while constant folding
+#?endif
+
                 # Don't constant fold the 'x' operator if the resulting string would be too big.
                 # 1024 is just a heuristic, measuring might show a bigger value would be fine.
                 if $all_args_known && $op.name eq '&infix:<x>' && $!symbols.is_from_core('&infix:<x>') {