[llvm] [Reassociate] Preserve NSW flags after expr tree rewriting (PR #93105)

[Akshay Deodhar via llvm-commits]

akshayrdeodhar wrote:

@dtcxzyw fixed formatting. 

I'll avoid rebasing from now on! I kept rebasing as a way to retrigger the build pipelines, there were some unrelated false failures (Flang, windows build failures), and I was trying to turn them green. 😂 

https://github.com/llvm/llvm-project/pull/93105