[llvm] [VPlan] Model branch cond to enter scalar epilogue in VPlan. (PR #92651)

[via llvm-commits]

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@@ -3481,6 +3424,18 @@ void InnerLoopVectorizer::fixVectorizedLoop(VPTransformState &State,
                                VF.getKnownMinValue() * UF);
 }
 
+// Helper to reorder blocks so they match the original order even after the
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ayalz wrote:

"FIXME: " can be added.

Unclear how this reordering works, even if temporarily to appease phi operand order:
if there are 2 predecessors it ensures LoopMiddleBlock is last.
if there are 3 predecessors A, B, C they will turn into B, C, A if A is LoopMiddleBlock and B, A, C otherwise?

https://github.com/llvm/llvm-project/pull/92651