[X86] Fix -Wunused-function in X86ISelLowering.cpp (NFC)

llvm-project/llvm/lib/Target/X86/X86ISelLowering.cpp:3582:13:
error: unused function 'isBlendOrUndef' [-Werror,-Wunused-function]
static bool isBlendOrUndef(ArrayRef<int> Mask) {
            ^
1 error generated.