[llvm] [IndVars] Generate zext nneg when locally obvious (PR #70823)

[Philip Reames via llvm-commits]

preames wrote:

I went ahead and landed this as c4b8f1e2b2aaf54eb74a2f40f999dc4c89b37234.  The change itself is pretty straight forward, and when attempting to rebase to resolve the test diff, git/github got into a confused state I couldn't figure out how to undo.  It didn't seem worth opening a separate PR and closing this one for something so straight forward.  

https://github.com/llvm/llvm-project/pull/70823