[llvm] [InstCombine] Drop NSW when converting `shl X, BW - 1` back into mul (PR #121633)

[Nikita Popov via llvm-commits]

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

LGTM, though I think it would be good to test more cases (you currently test the flag only on one operand, for one of the cases).

https://github.com/llvm/llvm-project/pull/121633