[llvm] [VPlan] Dispatch to multiple exit blocks via middle blocks. (PR #112138)

[David Sherwood via llvm-commits]

david-arm wrote:

> Updated, it required updating getUncountableEarlyExitingBlock to return nullptr in if !UncountableEarlyExit and some slight adjustments on how it is set (and clear the entries if it is false).
> 
> WDYT @david-arm ?

Sorry I only saw this now. It was hard to see amongst all the other comments. The original idea behind `getUncountableEarlyExitingBlock` was that you should only be calling this if you know you have an uncountable early exit. That's what the existing assert

```
    assert(getUncountableExitingBlocks().size() == 1 &&
           "Expected only a single uncountable exiting block");
```

was doing. It's pretty difficult to parse all of the comment history in github - is the reason for the change because you want to avoid calling `hasUncountableEarlyExit` and are doing this as a shortcut? That seems fine.

https://github.com/llvm/llvm-project/pull/112138