[llvm] [llvm] Fix fabs simplification (PR #152825)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Given `std::optional<bool>Res;`, `Res == false`, `Res == std::optional<bool>(false)`, and `Res && *Res == false` all mean the same thing.

https://github.com/llvm/llvm-project/pull/152825