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

[Nikita Popov via llvm-commits]

nikic 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?

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