[llvm] [MapVector] Introduce {keys, values} iterator, use in VPlan (PR #169675)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

Can you land ADT changes (w/ unit tests) separately from the use in vplan? This way potential reverts will be less disruptive 

https://github.com/llvm/llvm-project/pull/169675