[Vectorize] Fix warnings
This patch fixes:
third-party/unittest/googletest/include/gtest/gtest.h:1379:11:
error: comparison of integers of different signs: 'const unsigned
int' and 'const int' [-Werror,-Wsign-compare]
llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/InstrMaps.h:57:12:
error: unused variable 'Pair' [-Werror,-Wunused-variable]
