[llvm] [VPlan] Add exit phi operands during initial construction (NFC). (PR #136455)

[via llvm-commits]

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@@ -9388,11 +9387,8 @@ collectUsersInExitBlocks(Loop *OrigLoop, VPRecipeBuilder &Builder,
         continue;
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ayalz wrote:

Independent: is this early continue needed:
```
    // Nothing to do for unreachable exit blocks.
    if (ExitVPBB->getNumPredecessors() == 0)
      continue;
```
given that unreachable exit blocks have been emptied of their all their recipes - including phi ones?

https://github.com/llvm/llvm-project/pull/136455