Change explicit handling of impossible condition to call llvm_unreachable in IdempotentOperationChecker::PreVisitBinaryOperator.

llvm-svn: 107748
diff --git a/clang/lib/Checker/IdempotentOperationChecker.cpp b/clang/lib/Checker/IdempotentOperationChecker.cpp
index 646c6af..6ed1841 100644
--- a/clang/lib/Checker/IdempotentOperationChecker.cpp
+++ b/clang/lib/Checker/IdempotentOperationChecker.cpp
@@ -328,10 +328,10 @@
       case RHSis0:
         msg = "idempotent operation; the right operand is always 0";
         break;
-      case Impossible:
-        break;
       case Possible:
         llvm_unreachable("Operation was never marked with an assumption");
+      case Impossible:
+        llvm_unreachable(0);
       }
 
       // Create the SourceRange Arrays