[llvm] [indvars] Support known positive extends in getExtendedOperandRecurrence (PR #70990)

[Philip Reames via llvm-commits]

preames wrote:

> The test coverage here seems very weak. promote-iv-to-eliminate-casts.ll just has changes in instruction order, and for the LoopFlatten test it's not clear whether the change is from the adce use or the IV widening code.
> 
> Can you please add some explicit test coverage for this in IndVars?

Yeah, you were right here.  Check the most recent version, I added some tests to main and rebase this on top.  

https://github.com/llvm/llvm-project/pull/70990