Fix clang -Wsometimes-uninitialized warning in SCEV code

I don't believe this was a problem in practice, as it's likely that the
boolean wasn't checked unless the backend condition was non-null.

llvm-svn: 318073
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 7795a33..a643db8 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -4088,7 +4088,7 @@
 public:
   static const SCEV *rewrite(const SCEV *S, const Loop *L,
                              ScalarEvolution &SE) {
-    bool IsPosBECond;
+    bool IsPosBECond = false;
     Value *BECond = nullptr;
     if (BasicBlock *Latch = L->getLoopLatch()) {
       BranchInst *BI = dyn_cast<BranchInst>(Latch->getTerminator());