TruffleString.ToJavaStringNode: Add fast path for UTF-16 compatible strings.

Allows constant string optimizations, e.g. when comparing a converted constant TruffleString against a constant Java String.

Author: woess

truffle/src/com.oracle.truffle.api.strings/src/com/oracle/truffle/api/strings/TruffleString.java

@@ -6909,13 +6909,13 @@ static String doUTF16(TruffleString a,
             if (a.isEmpty()) {
                 return "";
             }
-            TruffleString cur = a.next;
-            if (cur != null) {
+            TruffleString cur;
+            if (a.isCompatibleToIntl(Encoding.UTF_16) || (cur = a.next) == null) {
+                cur = a;
+            } else {
                 while (cur != a && !cur.isCompatibleToIntl(Encoding.UTF_16)) {
                     cur = cur.next;
                 }
-            } else {
-                cur = a;
             }
             Encoding encodingA = Encoding.get(cur.encoding());
             final AbstractTruffleString utf16String;