[llvm] [InstCombine][ValueTracking] Improve logic for shift flags. (PR #67758)

[via llvm-commits]

goldsteinn wrote:

> The first commit looks good to me, please land it. For the last commit, what is the compile-time impact of the change, if any?

Minimal to none for the entire series: https://llvm-compile-time-tracker.com/compare.php?from=50ece4cba949787241b5fbfc94be6cfdc66e90ee&to=f0e676369e647a16acf7b1f6d2ecb17ec2dd388d&stat=instructions%3Au

https://github.com/llvm/llvm-project/pull/67758