[MergeICmps] Fix -Wsign-compare and typos (NFC)

/data/llvm-project/llvm/lib/Transforms/Scalar/MergeICmps.cpp:623:21: error: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigne
d long') [-Werror,-Wsign-compare]
  for (int i = 0; i < Comparisons.size(); i++) {
                  ~ ^ ~~~~~~~~~~~~~~~~~~
1 error generated.