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

[via llvm-commits]

https://github.com/ayalz commented:

This retains the single-exit(ing) (/ single implicit latch) property of regions, while fanning out from the single exiting to multiple (two) exit blocks, supporting a single early-exit together with the regular loop-terminating exit. The fanning out is modeled by one implicit construct to be converted eventually to explicit cascading if's. 

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