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

llvm-project/llvm/lib/Target/X86/X86MCInstLower.cpp:1588:48:
error: comparison of integers of different signs: 'unsigned int' and 'int' [-Werror,-Wsign-compare]
  if (C && C->getType()->getScalarSizeInBits() == SrcEltBits) {
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~
1 error generated.