[X86] Fix -Wsign-compare in X86MCInstLower.cpp (NFC)

llvm-project/llvm/lib/Target/X86/X86MCInstLower.cpp:1867:20:
 error: comparison of integers of different signs: 'int' and 'unsigned int' [-Werror,-Wsign-compare]
      if (SclWidth == C->getType()->getScalarSizeInBits()) {
          ~~~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.