[IndVars] Add an option to be able to disable LFTR

[skatkov]

Summary

This change adds an option disable-lftr to be able to disable Linear Function Test Replace optimization.
By default option is off so current behavior is not changed.

[sanjoy]

This LGTM.

If possible, please add a small test case.

[skatkov]

Sanjoy, is test ok?

lib/Transforms/Scalar/IndVarSimplify.cpp

cl::values(clEnumValN(NeverRepl, "never", "never replace exit value"),
                clEnumValN(AlwaysRepl, "always",
                           "always replace exit value whenever possible")));
 
 static cl::opt<bool> UsePostIncrementRanges(
   "indvars-post-increment-ranges", cl::Hidden,
   cl::desc("Use post increment control-dependent ranges in IndVarSimplify"),
   cl::init(true));
 
+static cl::opt<bool>
+DisableLFTR("disable-lftr", cl::Hidden, cl::init(false),
+            cl::desc("Disable Linear Function Test Replace optimization"));
+
 namespace {
 struct RewritePhi;
 
 class IndVarSimplify {
   LoopInfo *LI;
   ScalarEvolution *SE;
   DominatorTree *DT;
   const DataLayout &DL;
if (ReplaceExitValue != NeverRepl &&
       !isa<SCEVCouldNotCompute>(BackedgeTakenCount))
     rewriteLoopExitValues(L, Rewriter);
 
   // Eliminate redundant IV cycles.
   NumElimIV += Rewriter.replaceCongruentIVs(L, DT, DeadInsts);
 
   // If we have a trip count expression, rewrite the loop's exit condition
   // using it.  We can currently only handle loops with a single exit.
-  if (canExpandBackedgeTakenCount(L, SE, Rewriter) && needsLFTR(L, DT)) {
+  if (!DisableLFTR && canExpandBackedgeTakenCount(L, SE, Rewriter) &&
+      needsLFTR(L, DT)) {
     PHINode *IndVar = FindLoopCounter(L, BackedgeTakenCount, SE, DT);
     if (IndVar) {
       // Check preconditions for proper SCEVExpander operation. SCEV does not
       // express SCEVExpander's dependencies, such as LoopSimplify. Instead any
       // pass that uses the SCEVExpander must do it. This does not work well for
       // loop passes because SCEVExpander makes assumptions about all loops,
       // while LoopPassManager only forces the current loop to be simplified.
       //