[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.