[Polly] Use SmallDenseMap for MapInsnToMemAcc for better performance

Change MapInsnToMemAcc from std::map to SmallDenseMap<..., 16> for improved
compile time. This is consistent with the SmallDenseMap optimizations made
in DependenceAnalysis.

Note: Polly already has batch delinearization built into its architecture -
it collects all accesses in DetectionContext.Accesses, then processes them
together in getDelinearizationTerms() and hasBaseAffineAccesses(). This was
the model for the batch delinearization changes in DependenceAnalysis.

Author: sebpop

polly/include/polly/ScopDetection.h

@@ -48,6 +48,7 @@
 
 #include "polly/ScopDetectionDiagnostic.h"
 #include "polly/Support/ScopHelper.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/AliasSetTracker.h"
 #include "llvm/Analysis/RegionInfo.h"
@@ -77,6 +78,7 @@ using llvm::RegionInfo;
 using llvm::ScalarEvolution;
 using llvm::SCEVUnknown;
 using llvm::SetVector;
+using llvm::SmallDenseMap;
 using llvm::SmallSetVector;
 using llvm::SmallVectorImpl;
 using llvm::StringRef;
@@ -108,7 +110,7 @@ struct MemAcc {
       : Insn(I), Shape(S) {}
 };
 
-using MapInsnToMemAcc = std::map<const Instruction *, MemAcc>;
+using MapInsnToMemAcc = SmallDenseMap<const Instruction *, MemAcc, 16>;
 using PairInstSCEV = std::pair<const Instruction *, const SCEV *>;
 using AFs = std::vector<PairInstSCEV>;
 using BaseToAFs = std::map<const SCEVUnknown *, AFs>;