Fix unused variable warning. NFC. (llvm#2788)

(cherry picked from commit abacc52)

Author: shiltian

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -2743,7 +2743,7 @@ Instruction *InstCombinerImpl::visitBitCast(BitCastInst &CI) {
   if (DestTy == Src->getType())
     return replaceInstUsesWith(CI, Src);
 
-  if (FixedVectorType *DestVTy = dyn_cast<FixedVectorType>(DestTy)) {
+  if (isa<FixedVectorType>(DestTy)) {
     if (isa<IntegerType>(SrcTy)) {
       // If this is a cast from an integer to vector, check to see if the input
       // is a trunc or zext of a bitcast from vector.  If so, we can replace all