[llvm] [InstCombine] If inst in unreachable refers to an inst change it to poison (#65107) (PR #78444)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

Could you have a look at https://github.com/dtcxzyw/llvm-opt-benchmark/pull/226#discussion_r1487162964?
It is wired to replace an argument in a reachable invocation with a poison.

https://github.com/llvm/llvm-project/pull/78444