Revert "Remove unused variable (-Wunused)"

Patch that removed the use of this variable was  reverted in
8ade3d4

This reverts commit 3988a06.

llvm/lib/Transforms/Scalar/LoopInterchange.cpp

@@ -793,6 +793,7 @@ bool LoopInterchangeLegality::findInductionAndReductions(
 // This function indicates the current limitations in the transform as a result
 // of which we do not proceed.
 bool LoopInterchangeLegality::currentLimitations() {
+  BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
   BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
 
   // transform currently expects the loop latches to also be the exiting