[VPlan] Preserve DT (and SCEV) in VPlan-native path (#93287)

As a follow-up to b2f65e8, use the DTU to also update and preserve
the DT in the native path. This should also allow preserving SCEV in the
native path

PR: #93287

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7507,9 +7507,8 @@ LoopVectorizationPlanner::executePlan(
   LLVM_DEBUG(BestVPlan.dump());
 
   // Perform the actual loop transformation.
-  VPTransformState State(BestVF, BestUF, LI,
-                         EnableVPlanNativePath ? nullptr : DT, ILV.Builder,
-                         &ILV, &BestVPlan, OrigLoop->getHeader()->getContext());
+  VPTransformState State(BestVF, BestUF, LI, DT, ILV.Builder, &ILV, &BestVPlan,
+                         OrigLoop->getHeader()->getContext());
 
   // 0. Generate SCEV-dependent code into the preheader, including TripCount,
   // before making any changes to the CFG.
@@ -10370,16 +10369,9 @@ PreservedAnalyses LoopVectorizePass::run(Function &F,
         RemoveRedundantDbgInstrs(&BB);
     }
 
-    // We currently do not preserve dominator analyses with outer loop
-    // vectorization. Until this is addressed, mark these analyses as preserved
-    // only for non-VPlan-native path.
-    // TODO: Preserve Dominator analysis for VPlan-native path.
-    if (!EnableVPlanNativePath) {
-      PA.preserve<DominatorTreeAnalysis>();
-      PA.preserve<ScalarEvolutionAnalysis>();
-    }
-
     PA.preserve<LoopAnalysis>();
+    PA.preserve<DominatorTreeAnalysis>();
+    PA.preserve<ScalarEvolutionAnalysis>();
 
     if (Result.MadeCFGChange) {
       // Making CFG changes likely means a loop got vectorized. Indicate that

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -900,9 +900,9 @@ void VPlan::execute(VPTransformState *State) {
   }
 
   State->CFG.DTU.flush();
-  // DT is currently updated for non-native path only.
-  assert(EnableVPlanNativePath || State->CFG.DTU.getDomTree().verify(
-                                      DominatorTree::VerificationLevel::Fast));
+  assert(State->CFG.DTU.getDomTree().verify(
+             DominatorTree::VerificationLevel::Fast) &&
+         "DT not preserved correctly");
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)

llvm/test/Transforms/LoopVectorize/outer_loop_test1.ll

@@ -13,7 +13,7 @@
 ;   }
 ; }
 ;
-; RUN: opt -S -passes=loop-vectorize -enable-vplan-native-path -verify-loop-info < %s | FileCheck %s
+; RUN: opt -S -passes=loop-vectorize -enable-vplan-native-path -verify-loop-info -verify-dom-info < %s | FileCheck %s
 ; CHECK-LABEL: vector.ph:
 ; CHECK: %[[SplatVal:.*]] = insertelement <4 x i32> poison, i32 %n, i64 0
 ; CHECK: %[[Splat:.*]] = shufflevector <4 x i32> %[[SplatVal]], <4 x i32> poison, <4 x i32> zeroinitializer