[llvm] [InstCombine] Improve select equiv fold for plain condition (PR #83405)

[via llvm-commits]

vfdff wrote:

> @vfdff Could you please rebase this patch on main?

Thanks for the reminder. I found that the test cases have been committed in advance. (**commit bd9a2afa**)

https://github.com/llvm/llvm-project/pull/83405