[llvm] [llvm] Use llvm::less_first and llvm::less_second (NFC) (PR #136272)

[Vladimir Vereschaka via llvm-commits]

vvereschaka wrote:

@kazutakahirata ,

[llvm-clang-x86_64-expensive-checks-ubuntu](https://lab.llvm.org/buildbot/#/builders/187) builder also has broken with the following failed tests:
* LLVM::sched-group-barrier-pre-RA.mir
* LLVM::sched-group-barrier-pipeline-solver.mir

see more details here: https://lab.llvm.org/buildbot/#/builders/16/builds/17613/steps/6/logs/stdio

https://github.com/llvm/llvm-project/pull/136272