[TTI] Fix -Wsign-compare in BasicTTIImpl.h (NFC)

/llvm-project/llvm/include/llvm/CodeGen/BasicTTIImpl.h:948:33: error: comparison of integers of different signs: 'size_t' (aka 'unsigned long') and 'int' [-Werror,-Wsign-compare]
          (Index + Mask.size()) <= NumSrcElts) {
           ~~~~~~~~~~~~~~~~~~~  ^  ~~~~~~~~~~