[llvm] [VPlan] Add initial CFG simplification, removing BranchOnCond true. (PR #106748)

[via llvm-commits]

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@@ -3527,12 +3527,20 @@ class VPlan {
 
   /// Returns the 'middle' block of the plan, that is the block that selects
   /// whether to execute the scalar tail loop or the exit block from the loop
-  /// latch.
-  const VPBasicBlock *getMiddleBlock() const {
-    return cast<VPBasicBlock>(getScalarPreheader()->getPredecessors().front());
-  }
+  /// latch. If the scalar tail loop or exit block are known to always execute,
+  /// the middle block may branch directly to the block.
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ayalz wrote:

```suggestion
  /// latch. If the middle block is known to always proceed to one of these two blocks, it may branch to it unconditionally.
```
Also explain about the early-exit case, where middle.block is the 2nd successor of middle.split block, as depicted in https://llvm.org/docs/Vectorizers.html#early-exit-vectorization, which corresponds to the scalar preheader being absent from RegionSucc's successors? The middle block in this case conceptually has three successors: scalar preheader, latch.exit, early.exit with the first two postponed to be a successor's successors.

Note that if the middle block branches unconditionally to exit block (or scalar preheader block), the two blocks may subsequently be merged, causing RegionSucc to have no successors (or be the scalar preheader itself).

https://github.com/llvm/llvm-project/pull/106748