[Truffle] Array#push with an empty put non-null store is fine for compilation.

Author: chrisseaton

truffle/src/main/java/org/jruby/truffle/nodes/core/ArrayNodes.java

@@ -3352,7 +3352,6 @@ public RubyArray pushNullEmptyObjects(RubyArray array, Object... values) {
 
         @Specialization(guards = {"!isNull", "isEmpty"})
         public RubyArray pushEmptySingleIntegerFixnum(RubyArray array, Object... values) {
-            notDesignedForCompilation();
             // TODO CS 20-Apr-15 in reality might be better reusing any current storage, but won't worry about that for now
             array.setStore(values, values.length);
             return array;