[llvm] [IR] Add zext nneg flag (PR #67982)

[Nikita Popov via llvm-commits]

nikic wrote:

Just saw https://discourse.llvm.org/t/aggressive-conversion-of-sext-to-zext-blocks-indvarsimplify/61561/9?u=nikic, which proposed to call this `zext nsw` instead (and the equivalent sext would be `sext nuw`). I can definitely see the appeal in reusing existing nowrap flags, though it seems somewhat awkward in this instance, especially with one of the flags always being implied.

https://github.com/llvm/llvm-project/pull/67982