[NFC][LoopVectorize] Add more loop early exit asserts

This patch is split off llvm#120567, adding asserts in
addScalarResumePhis and addExitUsersForFirstOrderRecurrences
that the loop does not contain an uncountable early exit,
since the code cannot yet handle them correctly.

Author: david-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8925,8 +8925,9 @@ static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan) {
   VPTypeAnalysis TypeInfo(Plan.getCanonicalIV()->getScalarType());
   auto *ScalarPH = Plan.getScalarPreheader();
   auto *MiddleVPBB = cast<VPBasicBlock>(ScalarPH->getSinglePredecessor());
+  VPRegionBlock *VectorRegion = Plan.getVectorLoopRegion();
   VPBuilder VectorPHBuilder(
-      cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSinglePredecessor()));
+      cast<VPBasicBlock>(VectorRegion->getSinglePredecessor()));
   VPBuilder MiddleBuilder(MiddleVPBB, MiddleVPBB->getFirstNonPhi());
   VPBuilder ScalarPHBuilder(ScalarPH);
   VPValue *OneVPV = Plan.getOrAddLiveIn(
@@ -8958,6 +8959,8 @@ static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan) {
     // start value provides the value if the loop is bypassed.
     bool IsFOR = isa<VPFirstOrderRecurrencePHIRecipe>(VectorPhiR);
     auto *ResumeFromVectorLoop = VectorPhiR->getBackedgeValue();
+    assert(VectorRegion->getSingleSuccessor() == Plan.getMiddleBlock() &&
+           "Cannot handle loops with uncountable early exits");
     if (IsFOR)
       ResumeFromVectorLoop = MiddleBuilder.createNaryOp(
           VPInstruction::ExtractFromEnd, {ResumeFromVectorLoop, OneVPV}, {},
@@ -9127,6 +9130,9 @@ static void addExitUsersForFirstOrderRecurrences(
     if (!FOR)
       continue;
 
+    assert(VectorRegion->getSingleSuccessor() == Plan.getMiddleBlock() &&
+           "Cannot handle loops with uncountable early exits");
+
     // This is the second phase of vectorizing first-order recurrences, creating
     // extract for users outside the loop. An overview of the transformation is
     // described below. Suppose we have the following loop with some use after