Assert. Rather than silently stop printing.

llvm-svn: 28794
diff --git a/llvm/lib/CodeGen/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter.cpp
index 269b958..b3e27c9 100644
--- a/llvm/lib/CodeGen/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter.cpp
@@ -536,6 +536,8 @@
     break;
   case Type::PointerTyID:
     if (TD->getPointerSize() == 8) {
+      assert(Data64bitsDirective &&
+             "Target cannot handle 64-bit pointer exprs!");
       O << Data64bitsDirective;
       break;
     }