Bug fix in IRBuilder generating IR for an array literal

* We have to assign every elt build to a temporary and use the tmp
  in the array construction.

* This fixes the buggy IR generated by the builder for the snippet
  in issue 2574.

* But, this exposes a bug in OptTempVarsPass that is opting that
  away incorrectly. Will fix that in a separate patch and also add
  a test for it at that time.

core/src/main/java/org/jruby/ir/IRBuilder.java

@@ -782,7 +782,7 @@ public Operand buildAnd(final AndNode andNode) {
     public Operand buildArray(Node node) {
         List<Operand> elts = new ArrayList<>();
         for (Node e: node.childNodes())
-            elts.add(build(e));
+            elts.add(copyAndReturnValue(build(e)));
 
         return copyAndReturnValue(new Array(elts));
     }