Fix the interpreter crash that Michael McCracken found


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@17239 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/ExecutionEngine/ExecutionEngine.cpp b/lib/ExecutionEngine/ExecutionEngine.cpp
index 208f8e2..7fb4ad3 100644
--- a/lib/ExecutionEngine/ExecutionEngine.cpp
+++ b/lib/ExecutionEngine/ExecutionEngine.cpp
@@ -170,6 +170,7 @@
 /// 
 GenericValue ExecutionEngine::getConstantValue(const Constant *C) {
   GenericValue Result;
+  if (isa<UndefValue>(C)) return Result;
 
   if (ConstantExpr *CE = const_cast<ConstantExpr*>(dyn_cast<ConstantExpr>(C))) {
     switch (CE->getOpcode()) {