[llvm] [VPlan] Retain exit conditions and edges in initial VPlan (NFC). (PR #137709)

[via llvm-commits]

================
@@ -425,21 +409,31 @@ static void createLoopRegion(VPlan &Plan, VPBlockBase *HeaderVPB) {
   VPBlockUtils::disconnectBlocks(PreheaderVPBB, HeaderVPB);
   VPBlockUtils::disconnectBlocks(LatchVPBB, HeaderVPB);
   VPBlockBase *Succ = LatchVPBB->getSingleSuccessor();
-  assert(LatchVPBB->getNumSuccessors() <= 1 &&
-         "Latch has more than one successor");
-  if (Succ)
-    VPBlockUtils::disconnectBlocks(LatchVPBB, Succ);
+  assert(Succ && "Latch expected to be left with a single successor");
+
+  // Use a temporary placeholder between LatchVPBB and its successor, to
+  // preserve the original predecessor/successor order of the blocks.
+  auto *PlaceHolder = Plan.createVPBasicBlock("Region place holder");
+  VPBlockUtils::insertOnEdge(LatchVPBB, Succ, PlaceHolder);
+  VPBlockUtils::disconnectBlocks(LatchVPBB, PlaceHolder);
+  VPBlockUtils::connectBlocks(PreheaderVPBB, PlaceHolder);
 
   auto *R = Plan.createVPRegionBlock(HeaderVPB, LatchVPBB, "",
                                      false /*isReplicator*/);
-  // All VPBB's reachable shallowly from HeaderVPB belong to top level loop,
-  // because VPlan is expected to end at top level latch disconnected above.
+  // All VPBB's reachable shallowly from HeaderVPB belong to the current region,
+  // except the exit blocks reachable via non-latch exiting blocks,
+  SmallPtrSet<VPBlockBase *, 2> ExitBlocks(Plan.getExitBlocks().begin(),
+                                           Plan.getExitBlocks().end());
   for (VPBlockBase *VPBB : vp_depth_first_shallow(HeaderVPB))
-    VPBB->setParent(R);
+    if (!ExitBlocks.contains(VPBB))
+      VPBB->setParent(R);
 
   VPBlockUtils::insertBlockAfter(R, PreheaderVPBB);
-  if (Succ)
-    VPBlockUtils::connectBlocks(R, Succ);
+  VPBlockUtils::insertOnEdge(PlaceHolder, Succ, R);
+
+  // Remove placeholder block.
+  VPBlockUtils::disconnectBlocks(R, PlaceHolder);
+  VPBlockUtils::disconnectBlocks(PlaceHolder, R);
----------------
ayalz wrote:

Inserting R first after Preheader (clearer to instead explicitly insert R on the edge from Preheader to PlaceHolder?) and then inserting R again on the edge between PlaceHolder and Succ seems a bit confusing, thereby creating bidirectional edges between R and PlaceHolder which are then removed. It's like inserting R both before PlaceHolder and after it, but both these insertions require R to be disconnected. In essence we want to replace PlaceHolder with R.
Another alternative is to have an empty region as a placeholder rather than an empty basic block, and then move/copy R's entry and exit blocks into it:
```
  auto *PlaceHolder = Plan.createVPRegionBlock("", false /*isReplicator*/);
```
followed by
```
  PlaceHolder.setEntry(R.getEntry());
  PlaceHolder.setExit(R.getExit());
```
essentially turning `R` to be the temporary block and `PlaceHolder` to be `R`, constructed outside in rather than inside out?

https://github.com/llvm/llvm-project/pull/137709