[llvm] [VPlan] Refactor VPlan creation, add transform introducing region (NFC). (PR #128419)

[via llvm-commits]

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@@ -9312,14 +9312,15 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
             return !CM.requiresScalarEpilogue(VF.isVector());
           },
           Range);
-  VPlanPtr Plan = VPlan::createInitialVPlan(Legal->getWidestInductionType(),
-                                            PSE, RequiresScalarEpilogueCheck,
-                                            CM.foldTailByMasking(), OrigLoop);
-
+  auto Plan = std::make_unique<VPlan>(OrigLoop);
   // Build hierarchical CFG.
   VPlanHCFGBuilder HCFGBuilder(OrigLoop, LI, *Plan);
   HCFGBuilder.buildHierarchicalCFG();
 
+  VPlanTransforms::introduceTopLevelVectorLoopRegion(
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ayalz wrote:

Should this introduce all regions, i.e., lifting a flat CFG into a hierarchical one? With the inverse lowing conversion taking place at the end, to simplify code-gen.

https://github.com/llvm/llvm-project/pull/128419