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

[via llvm-commits]

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@@ -448,11 +448,13 @@ VPBasicBlock::createEmptyBasicBlock(VPTransformState::CFGState &CFG) {
 }
 
 void VPIRBasicBlock::execute(VPTransformState *State) {
-  assert(getHierarchicalSuccessors().empty() &&
-         "VPIRBasicBlock cannot have successors at the moment");
-
   State->Builder.SetInsertPoint(getIRBasicBlock()->getTerminator());
   executeRecipes(State, getIRBasicBlock());
+  if (getSingleSuccessor() && isa<UnreachableInst>(getIRBasicBlock()->getTerminator())) {
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ayalz wrote:

If terminator is not UnreachableInst then it was already backpatched - is that possible? Should this check be an assert.

// Replace the terminating UnreachableInst with an unconditional branch set to null (explicitly as constructor accepts only non-null destinations), to be reset when executing the successor.

Replacing one stub with another ... would having the successor take care of this replacement, or creating a branch-to-null originally instead of unreachable, be better.

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