[640699] Reduced compound expressions need explicit conversion too.

mcs/mcs/assign.cs

@@ -654,6 +654,9 @@ namespace Mono.CSharp {
 			// Otherwise, if the selected operator is a predefined operator
 			//
 			Binary b = source as Binary;
+			if (b == null && source is ReducedExpression)
+				b = ((ReducedExpression) source).OriginalExpression as Binary;
+
 			if (b != null) {
 				//
 				// 2a. the operator is a shift operator

mcs/mcs/ecore.cs

@@ -1873,6 +1873,16 @@ namespace Mono.CSharp {
 			this.loc = orig_expr.Location;
 		}
 
+		#region Properties
+
+		public Expression OriginalExpression {
+			get {
+				return orig_expr;
+			}
+		}
+
+		#endregion
+
 		//
 		// Creates fully resolved expression switcher
 		//