[llvm] [VPlan] Add BranchOnMultiCond, use for early exit plans. (PR #172750)

[via llvm-commits]

================
@@ -391,20 +391,21 @@ static bool hasDuplicates(const SmallVectorImpl<VPBlockBase *> &VPBlockVec) {
 bool VPlanVerifier::verifyBlock(const VPBlockBase *VPB) {
   auto *VPBB = dyn_cast<VPBasicBlock>(VPB);
   // Check block's condition bit.
-  if (!isa<VPIRBasicBlock>(VPB)) {
+  if (VPBB && !isa<VPIRBasicBlock>(VPB)) {
+    // VPRegionBlocks can have multiple successors (e.g., with
+    // BranchOnTwoConds) without needing a terminator, so only check
+    // VPBasicBlocks.
     if (VPB->getNumSuccessors() > 1 ||
-        (VPBB && VPBB->getParent() && VPBB->isExiting() &&
+        (VPBB->getParent() && VPBB->isExiting() &&
          !VPBB->getParent()->isReplicator())) {
----------------
ayalz wrote:

ok, but a similar
```
if (VPBB->getNumSuccessors() == 2 ||
      (VPBB->isExiting() && !VPBB->getParent()->isReplicator())) {
```
also appears in `hasConditionalTerminator()` above, hence the thought of providing something common like
```
  unsigned getNumCFGSuccessors(VPBB) {
    unsigned NumSuccessors = VPBB->getNumSuccessors();
    if (!VPBB->isExiting())
      return NumSuccessors;
    // NumSuccessors is zero, need to consider parent's successors instead.
    auto *Parent = VPBB->getParent();
    NumSuccessors = Parent->getNumSuccessors();
    // Account for implicit backedge.
    if (!Parent->isReplicator())
      ++NumSuccessors;
    return NumSuccessors;
  }
```
somewhat consistent with `getCFGPredecessor(Index)`.
Anyhow, can be done independently.

https://github.com/llvm/llvm-project/pull/172750