Avoid bad assertion

llvm-svn: 4918
diff --git a/llvm/lib/Target/X86/Printer.cpp b/llvm/lib/Target/X86/Printer.cpp
index 6206059..ae14e41 100644
--- a/llvm/lib/Target/X86/Printer.cpp
+++ b/llvm/lib/Target/X86/Printer.cpp
@@ -183,7 +183,7 @@
     assert(isReg(MI->getOperand(0)) &&
            (MI->getNumOperands() == 1 || 
             (MI->getNumOperands() == 2 &&
-             (MI->getOperand(1).getVRegValue() ||
+             (MI->getOperand(1).getVRegValueOrNull() ||
               isImmediate(MI->getOperand(1))))) &&
            "Illegal form for AddRegFrm instruction!");