[llvm] [Reassociate] Preserve NUW flags after expr tree rewriting (PR #72360)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

Sorry, it doesn't hold for `mul` if one of the operands is zero.
Alive2: https://alive2.llvm.org/ce/z/686-w6

https://github.com/llvm/llvm-project/pull/72360