[llvm] [Transforms][Utils][PromoteMem2Reg] Propagate nnan flag on par with the nsz flag (PR #114271)

[Matt Arsenault via llvm-commits]

arsenm wrote:

Can you add a phase ordering test that shows what case this helps? The nsz case was more difficult to recover; I think nnan/ninf are simpler. 

https://github.com/llvm/llvm-project/pull/114271