[llvm] [VPlan] Add BranchOnMultiCond, use for early exit plans. (PR #172750)

[via llvm-commits]

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ayalz wrote:

Worth adding a dotted circular arrow from vector loop to itself, to highlight the 3 successors of its terminating BranchOnToConds?

https://github.com/llvm/llvm-project/pull/172750