[X86] Fix -Wsign-compare in X86ISelLowering.cpp (NFC)

llvm-project/llvm/lib/Target/X86/X86ISelLowering.cpp:40081:21:
error: comparison of integers of different signs: 'int' and 'unsigned int' [-Werror,-Wsign-compare]
  for (int I = 0; I != NumElts; ++I) {
                  ~ ^  ~~~~~~~
1 error generated.