Add NOT_YET_COMPUTED_IDENTITY_HASH_CODE marker.

Author: cstancu

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/meta/DirectSubstrateObjectConstant.java

@@ -79,7 +79,7 @@ public SubstrateObjectConstant uncompress() {
 
     @Override
     public int getIdentityHashCode() {
-        if (identityHashCode == 0) {
+        if (identityHashCode == NOT_YET_COMPUTED_IDENTITY_HASH_CODE) {
             VMError.guarantee(!SubstrateUtil.HOSTED);
             identityHashCode = computeIdentityHashCode(object);
         }

substratevm/src/com.oracle.svm.core/src/com/oracle/svm/core/meta/SubstrateObjectConstant.java

@@ -35,16 +35,18 @@
 import jdk.vm.ci.meta.VMConstant;
 
 public abstract class SubstrateObjectConstant implements JavaConstant, TypedConstant, CompressibleConstant, VMConstant {
+    protected static final int NOT_YET_COMPUTED_IDENTITY_HASH_CODE = -1;
+
     public static JavaConstant forObject(Object object) {
-        return forObject(object, false, 0);
+        return forObject(object, false, NOT_YET_COMPUTED_IDENTITY_HASH_CODE);
     }
 
     public static JavaConstant forObject(Object object, int identityHashCode) {
         return forObject(object, false, identityHashCode);
     }
 
     public static JavaConstant forObject(Object object, boolean compressed) {
-        return forObject(object, compressed, 0);
+        return forObject(object, compressed, NOT_YET_COMPUTED_IDENTITY_HASH_CODE);
     }
 
     public static JavaConstant forObject(Object object, boolean compressed, int identityHashCode) {