[llvm] [X86] Resolve FIXME: Accept live flag if no overflow occurs (PR #86836)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

You're going to need a lot of tests for all of this :) Should almost be split into seperate patches

https://github.com/llvm/llvm-project/pull/86836