Fix PR7096. When a block containing multiple defs is tail duplicated, the

SSAUpdater for the value from the first def may see uses of undefined values,
because the later defs will not have been updated yet.

llvm-svn: 103407

lib/CodeGen/MachineSSAUpdater.cpp

@@ -329,7 +329,7 @@ public:
   /// InstrIsPHI - Check if an instruction is a PHI.
   ///
   static MachineInstr *InstrIsPHI(MachineInstr *I) {
-    if (I->isPHI())
+    if (I && I->isPHI())
       return I;
     return 0;
   }