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

[Paul Walker via cfe-commits]

https://github.com/paulwalker-arm approved this pull request.

The change looks reasonable to me and consistent within an existing solution to a similar problem.  Worse case, the new code should be removed when we're at a point that instruction fast math flags are sufficiently preserved/propagated.

That said, @pawosm-arm please wait until middle of next week before landing the PR just in case anybody strongly disagrees with this path.

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