[LV] Allow forcing tail folding when constructing the cost model (WIP).

[fhahn]

Summary

This may help to simplify D142015 in combination with D142669.

[fhahn]

Fix some crashes.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

 /// vectorization.
 /// In many cases vectorization is not profitable. This can happen because of
 /// a number of reasons. In this class we mainly attempt to predict the
 /// expected speedup/slowdowns due to the supported instruction set. We use the
 /// TargetTransformInfo to query the different backends for the cost of
 /// different operations.
 class LoopVectorizationCostModel {
 public:
-  LoopVectorizationCostModel(ScalarEpilogueLowering SEL, Loop *L,
-                             PredicatedScalarEvolution &PSE, LoopInfo *LI,
-                             LoopVectorizationLegality *Legal,
-                             const TargetTransformInfo &TTI,
-                             const TargetLibraryInfo *TLI, DemandedBits *DB,
-                             AssumptionCache *AC,
-                             OptimizationRemarkEmitter *ORE, const Function *F,
-                             const LoopVectorizeHints *Hints,
-                             InterleavedAccessInfo &IAI)
+  LoopVectorizationCostModel(
+      ScalarEpilogueLowering SEL, Loop *L, PredicatedScalarEvolution &PSE,
+      LoopInfo *LI, LoopVectorizationLegality *Legal,
+      const TargetTransformInfo &TTI, const TargetLibraryInfo *TLI,
+      DemandedBits *DB, AssumptionCache *AC, OptimizationRemarkEmitter *ORE,
+      const Function *F, const LoopVectorizeHints *Hints,
+      InterleavedAccessInfo &IAI, bool ForceFoldTailByMasking = false)
       : ScalarEpilogueStatus(SEL), TheLoop(L), PSE(PSE), LI(LI), Legal(Legal),
         TTI(TTI), TLI(TLI), DB(DB), AC(AC), ORE(ORE), TheFunction(F),
-        Hints(Hints), InterleaveInfo(IAI) {}
+        Hints(Hints), InterleaveInfo(IAI),
+        FoldTailByMasking(ForceFoldTailByMasking) {}
 
   /// \return An upper bound for the vectorization factors (both fixed and
   /// scalable). If the factors are 0, vectorization and interleaving should be
   /// avoided up front.
   FixedScalableVFPair computeMaxVF(ElementCount UserVF, unsigned UserIC);
 
   /// \return True if runtime checks are required for vectorization, and false
   /// otherwise.
private:
   /// least once. This may be needed as a fallback loop in case runtime
   /// aliasing/dependence checks fail, or to handle the tail/remainder
   /// iterations when the trip count is unknown or doesn't divide by the VF,
   /// or as a peel-loop to handle gaps in interleave-groups.
   /// Under optsize and when the trip count is very small we don't allow any
   /// iterations to execute in the scalar loop.
   ScalarEpilogueLowering ScalarEpilogueStatus = CM_ScalarEpilogueAllowed;
 
-  /// All blocks of loop are to be masked to fold tail of scalar iterations.
-  bool FoldTailByMasking = false;
-
   /// A map holding scalar costs for different vectorization factors. The
   /// presence of a cost for an instruction in the mapping indicates that the
   /// instruction will be scalarized when vectorizing with the associated
   /// vectorization factor. The entries are VF-ScalarCostTy pairs.
   DenseMap<ElementCount, ScalarCostsTy> InstsToScalarize;
 
   /// Holds the instructions known to be uniform after vectorization.
   /// The data is collected per VF.
public:
 
   /// Loop Vectorize Hint.
   const LoopVectorizeHints *Hints;
 
   /// The interleave access information contains groups of interleaved accesses
   /// with the same stride and close to each other.
   InterleavedAccessInfo &InterleaveInfo;
 
+  /// All blocks of loop are to be masked to fold tail of scalar iterations.
+  bool FoldTailByMasking;
+
   /// Values to ignore in the cost model.
   SmallPtrSet<const Value *, 16> ValuesToIgnore;
 
   /// Values to ignore in the cost model when VF > 1.
   SmallPtrSet<const Value *, 16> VecValuesToIgnore;
 
   /// All element types found in the loop.
   SmallPtrSet<Type *, 16> ElementTypesInLoop;