[LoopFlatten] Add statistic for number of loops flattened. NFC

[dmgreen]

Sounds useful. LGTM.

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

 // expression like i*M+j. If they had any other uses, we would have to insert a
 // div/mod to reconstruct the original values, so this wouldn't be profitable.
 //
 // We also need to prove that N*M will not overflow.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Scalar/LoopFlatten.h"
+
+#include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/Verifier.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
 #include "llvm/Transforms/Utils/SimplifyIndVar.h"
 
-#define DEBUG_TYPE "loop-flatten"
-
 using namespace llvm;
 using namespace llvm::PatternMatch;
 
+#define DEBUG_TYPE "loop-flatten"
+
+STATISTIC(NumFlattened, "Number of loops flattened");
+
 static cl::opt<unsigned> RepeatedInstructionThreshold(
     "loop-flatten-cost-threshold", cl::Hidden, cl::init(2),
     cl::desc("Limit on the cost of instructions that can be repeated due to "
              "loop flattening"));
 
 static cl::opt<bool>
     AssumeNoOverflow("loop-flatten-assume-no-overflow", cl::Hidden,
                      cl::init(false),
for (Value *V : FI.LinearIVUses) {
     V->replaceAllUsesWith(OuterValue);
   }
 
   // Tell LoopInfo, SCEV and the pass manager that the inner loop has been
   // deleted, and any information that have about the outer loop invalidated.
   SE->forgetLoop(FI.OuterLoop);
   SE->forgetLoop(FI.InnerLoop);
   LI->erase(FI.InnerLoop);
+
+  // Increment statistic value.
+  NumFlattened++;
+
   return true;
 }
 
 static bool CanWidenIV(FlattenInfo &FI, DominatorTree *DT, LoopInfo *LI,
                        ScalarEvolution *SE, AssumptionCache *AC,
                        const TargetTransformInfo *TTI) {
   if (!WidenIV) {
     LLVM_DEBUG(dbgs() << "Widening the IVs is disabled\n");