Use enum as case value if possible.

ICSharpCode.Decompiler/Ast/AstMethodBodyBuilder.cs

@@ -123,7 +123,7 @@ namespace ICSharpCode.Decompiler.Ast
 				SwitchStatement switchStmt = new SwitchStatement() { Expression = (Expression)TransformExpression(ilSwitch.Condition) };
 				foreach (var caseBlock in ilSwitch.CaseBlocks) {
 					SwitchSection section = new SwitchSection();
-					section.CaseLabels.AddRange(caseBlock.Values.Select(i => new CaseLabel() { Expression = new PrimitiveExpression(i) }));
+					section.CaseLabels.AddRange(caseBlock.Values.Select(i => new CaseLabel() { Expression = AstBuilder.MakePrimitive(i, ilSwitch.Condition.InferredType) }));
 					section.Statements.Add(TransformBlock(caseBlock));
 					switchStmt.SwitchSections.Add(section);
 				}