[llvm] [LV] Add a statistic for early exit vectorization (PR #145730)

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annamthomas wrote:

> perhaps for interleave groups which require the scalar epilogue?)

Yep, it is failing for exactly that. What's happening is that we are vectorizing the scalar epilog loop and there are no exit blocks for that scalar epilog loop. So, we fail. 
It's not really the part of the statistic change, so I'll place it as a separate commit in this same MR. Hope that is okay.


https://github.com/llvm/llvm-project/pull/145730