[X86] Fix warning about unused variable [NFC]

Without this gcc complains like
 ../lib/Target/X86/X86FixupVectorConstants.cpp:70:13: warning: unused variable 'CUndef' [-Wunused-variable]
    70 |   if (auto *CUndef = dyn_cast<UndefValue>(C))
       |             ^~~~~~

Remove the unused variable and change dyn_cast to isa.

Author: mikaelholmen

llvm/lib/Target/X86/X86FixupVectorConstants.cpp

@@ -67,7 +67,7 @@ FunctionPass *llvm::createX86FixupVectorConstants() {
 static std::optional<APInt> extractConstantBits(const Constant *C) {
   unsigned NumBits = C->getType()->getPrimitiveSizeInBits();
 
-  if (auto *CUndef = dyn_cast<UndefValue>(C))
+  if (isa<UndefValue>(C))
     return APInt::getZero(NumBits);
 
   if (auto *CInt = dyn_cast<ConstantInt>(C))