Remove dead code. Improve llvm_unreachable text. Simplify some control flow.

llvm-svn: 150918
diff --git a/llvm/lib/ExecutionEngine/Interpreter/Execution.cpp b/llvm/lib/ExecutionEngine/Interpreter/Execution.cpp
index ac4c49f..3dce3b3 100644
--- a/llvm/lib/ExecutionEngine/Interpreter/Execution.cpp
+++ b/llvm/lib/ExecutionEngine/Interpreter/Execution.cpp
@@ -1235,7 +1235,7 @@
     break;
   default:
     dbgs() << "Unhandled ConstantExpr: " << *CE << "\n";
-    llvm_unreachable(0);
+    llvm_unreachable("Unhandled ConstantExpr");
   }
   return Dest;
 }