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

[Florian Hahn via llvm-commits]

fhahn wrote:

@annamthomas thanks for landing the fix! Rebased, also the majority of the pattern-match changes have been split up and submitted & used separately.

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