MemCmpExpansion::getCompareLoadPairs - assert we find a comparison diff. NFCI.

Fix scan-build uninitialized warning and assert the final diff isn't null.

llvm-svn: 361095
diff --git a/llvm/lib/CodeGen/ExpandMemCmp.cpp b/llvm/lib/CodeGen/ExpandMemCmp.cpp
index a3a9790..06c171a 100644
--- a/llvm/lib/CodeGen/ExpandMemCmp.cpp
+++ b/llvm/lib/CodeGen/ExpandMemCmp.cpp
@@ -323,7 +323,7 @@
   assert(LoadIndex < getNumLoads() &&
          "getCompareLoadPairs() called with no remaining loads");
   std::vector<Value *> XorList, OrList;
-  Value *Diff;
+  Value *Diff = nullptr;
 
   const unsigned NumLoads =
       std::min(getNumLoads() - LoadIndex, NumLoadsPerBlockForZeroCmp);
@@ -400,6 +400,8 @@
     while (OrList.size() != 1) {
       OrList = pairWiseOr(OrList);
     }
+
+    assert(Diff && "Failed to find comparison diff");
     Cmp = Builder.CreateICmpNE(OrList[0], ConstantInt::get(Diff->getType(), 0));
   }