[X86] Fix -Wunused-function warning (NFC)

GCC warning:
```
/llvm-project/llvm/lib/Target/X86/X86ISelLowering.cpp:9212:13: warning: ‘bool isHorizOp(unsigned int)’ defined but not used [-Wunused-function]
 9212 | static bool isHorizOp(unsigned Opcode) {
      |             ^~~~~~~~~
```