[llvm] [IndVarSimplify] Fix poison-safety when reusing instructions (PR #80458)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM

I'm mildly concerned about compile time impact here, but since this is a correctness fix, I'm fine landing it and revisiting if the compile time is problematic in practice.  

https://github.com/llvm/llvm-project/pull/80458