[llvm] update_test_checks: collect original check lines for old versions of lit tests (PR #111148)

[Nikita Popov via llvm-commits]

nikic wrote:

Tried regenerating all InstCombine tests again, and now only 44 files change, and it looks like for all of these the changes are not due to instruction naming, but other formatting changes. So this seems to work perfectly now!

https://github.com/llvm/llvm-project/pull/111148