[PowerPC] Fix gcc warning about unused variable [NFC]

gcc warned about
../lib/Target/PowerPC/PPCTargetTransformInfo.cpp:1401:13: warning: unused variable 'VecTy' [-Wunused-variable]
 1401 |   if (auto *VecTy = dyn_cast<FixedVectorType>(DataType)) {
      |             ^~~~~