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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107748 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Checker/IdempotentOperationChecker.cpp b/lib/Checker/IdempotentOperationChecker.cpp
index 646c6af..6ed1841 100644
--- a/lib/Checker/IdempotentOperationChecker.cpp
+++ b/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