[llvm] [VPlan] Replace disjoint or with add instead of dropping disjoint. (PR #83821)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

Looks reasonable conceptually, but I'm not the right person to review VPlan changes.

Test case seems to be missing?

https://github.com/llvm/llvm-project/pull/83821