Fix #310 - nullable conversion with ignored result

It should also help a bit other conversions with ignored result -
whenever we know what the conversion will always pass, we
don't need to emit any code. In some cases, this may save a boxing
conversion (and it will probably save some work for the JIT)

Author: exyi

src/FastExpressionCompiler/FastExpressionCompiler.cs

@@ -2763,6 +2763,12 @@ private static bool TryEmitConvert(UnaryExpression expr, IReadOnlyList<PE> param
                 var underlyingNullableSourceType = Nullable.GetUnderlyingType(sourceType);
                 var targetType = expr.Type;
 
+                if (targetType.IsAssignableFrom(sourceType) && (parent & ParentFlags.IgnoreResult) != 0)
+                {
+                    // quick path for ignored result & conversion which can't cause exception: just do nothing
+                    return TryEmit(opExpr, paramExprs, il, ref closure, setup, parent);
+                }
+
                 if (sourceTypeIsNullable && targetType == underlyingNullableSourceType)
                 {
                     if (!TryEmit(opExpr, paramExprs, il, ref closure, setup, 
@@ -2813,9 +2819,7 @@ private static bool TryEmitConvert(UnaryExpression expr, IReadOnlyList<PE> param
                     if (method != null && method.DeclaringType == targetType && method.GetParameters()[0].ParameterType == sourceType)
                     {
                         il.Emit(OpCodes.Call, method);
-                        if ((parent & ParentFlags.IgnoreResult) != 0)
-                            il.Emit(OpCodes.Pop);
-                        return true;
+                        return il.EmitPopIfIgnoreResult(parent);
                     }
 
                     var actualSourceType = sourceTypeIsNullable ? underlyingNullableSourceType : sourceType;

test/FastExpressionCompiler.TestsRunner.Net472/Program.cs

@@ -201,6 +201,9 @@ void Run(Func<int> run, string name = null)
                 Run(new Issue308_Wrong_delegate_type_returned_with_closure().Run);
                 Run(new FastExpressionCompiler.LightExpression.IssueTests.Issue308_Wrong_delegate_type_returned_with_closure().Run);
 
+                Run(new Issue310_InvalidProgramException_ignored_nullable().Run);
+                Run(new FastExpressionCompiler.LightExpression.IssueTests.Issue310_InvalidProgramException_ignored_nullable().Run);
+
                 Console.WriteLine($"============={Environment.NewLine}IssueTests are passing in {sw.ElapsedMilliseconds} ms.");
             });