Fix for small problems appeared after some PR merges (#330)

PR 237 - Issue with bulk operation translation for UPDATE. Wrong IF condition
PR 265 - Issue with displaying Provider instance while debugging. Wrong ref to Sources.Count
in DebuggerDisplay attribute

Author: alex-kulakov

Extensions/Xtensive.Orm.BulkOperations/Internals/SetOperation.cs

@@ -136,7 +136,7 @@ private void AddComputedStaticExpression(AddValueContext addContext)
       var sqlSelect = request.Query;
       var exp = sqlSelect.OrderBy[0].Expression;
 
-      if (exp is SqlPlaceholder placeholder) {
+      if (exp is not SqlPlaceholder placeholder) {
         parent.Bindings.AddRange(request.ParameterBindings);
         addContext.Values.Add(column, exp);
         return;

Orm/Xtensive.Orm/Orm/Rse/Providers/Provider.cs

@@ -17,7 +17,7 @@ namespace Xtensive.Orm.Rse.Providers
   /// Abstract base class for any query provider.
   /// </summary>
   [Serializable]
-  [DebuggerDisplay("{DebuggerDisplayName}, Source count = {Sources.Length}")]
+  [DebuggerDisplay("{DebuggerDisplayName}, Source count = {Sources.Count}")]
   public abstract class Provider
   {
     private const string ToString_ProviderTypeSuffix = "Provider";