[llvm] LiveRangeEdit: Clear all dead flags when rematerializing (PR #73933)

[Krzysztof Parzyszek via llvm-commits]

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

This should be safe to do, even if we delete `dead` flag on an actually dead def.  I'm not sure if we would rematerialize an instruction with multiple defs, but in principle doing that should be ok.

https://github.com/llvm/llvm-project/pull/73933