Simplify code. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189168 91177308-0d34-0410-b5e6-96231b3b80d8
Add a function object to compare the first or second component of a std::pair.

Replace instances of this scattered around the code base.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189169 91177308-0d34-0410-b5e6-96231b3b80d8