Avoid re-translating NhLinqExpression.

NhLinqExpression instances may be translated multiple times in two cases:
 * When they have list parameters
 * When their plan is not cacheable, since it is looked-up two times in
   cache per execution, and (re)generated in case of cache miss.

Author: fredericDelaporte

src/NHibernate/Impl/ExpressionQueryImpl.cs

@@ -74,11 +74,15 @@ protected override IQueryExpression ExpandParameters(IDictionary<string, TypedVa
 			}
 
 			//TODO: Do we need to translate expression one more time here?
+			// This is not much an issue anymore: ExpressionQueryImpl are currently created only with NhLinqExpression
+			// which do cache their translation. By the way, the false transmitted below as filter parameter to
+			// Translate is wrong, since this ExpandParameters may also be called from ExpressionFilterImpl.
+			// But that does not matter for NhLinqExpression.
 			var newTree = ParameterExpander.Expand(QueryExpression.Translate(Session.Factory, false), map);
 			var key = new StringBuilder(QueryExpression.Key);
 
 			foreach (var pair in map)
-							   {
+			{
 				key.Append(' ');
 				key.Append(pair.Key);
 				key.Append(':');

src/NHibernate/Linq/NhLinqExpression.cs

@@ -68,6 +68,14 @@ public NhLinqExpression(Expression expression, ISessionFactoryImplementor sessio
 
 		public IASTNode Translate(ISessionFactoryImplementor sessionFactory, bool filter)
 		{
+			if (ExpressionToHqlTranslationResults != null)
+			{
+				// Query has already been translated. Arguments do not really matters, because queries are anyway tied
+				// to a single session factory and cannot switch from being a filter query (query on mapped collection)
+				// or not.
+				return ExpressionToHqlTranslationResults.Statement.AstNode;
+			}
+
 			var requiredHqlParameters = new List<NamedParameterDescriptor>();
 			var queryModel = NhRelinqQueryParser.Parse(_expression);
 			var visitorParameters = new VisitorParameters(sessionFactory, _constantToParameterMap, requiredHqlParameters,