Revert "Remove unnecessary null check. NFC."

This reverts commit r243167.

Duncan pointed out that dyn_cast can return null in these cases, so this
was an unsafe commit to make.  Sorry for the noise.

Worryingly there were no tests which fail...

llvm-svn: 243302

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -817,6 +817,9 @@ bool LoopInterchangeLegality::currentLimitations() {
     InnerIndexVarInc =
         dyn_cast<Instruction>(InnerInductionVar->getIncomingValue(0));
 
+  if (!InnerIndexVarInc)
+    return true;
+
   // Since we split the inner loop latch on this induction variable. Make sure
   // we do not have any instruction between the induction variable and branch
   // instruction.