[llvm] [InstCombine] Return `nsw` when convert `shl` to `mul` if its on the RHS (PR #121671)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

No strong opinion, but I'd leave this alone. The current code is obvious (we're just converting shl to mul and determine whether the mul has the nsw flag), while this would need some explanation for why it's okay to keep the nsw in one case but not the other.

https://github.com/llvm/llvm-project/pull/121671