[LoopFlatten] Use Loop to identify loop induction phi. NFC

[RosieSumpter]

Summary

Replace code which identifies induction phi with helper function
getInductionVariable to improve robustness.

[SjoerdMeijer]

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

126–127

We don't need this anymore?

131

And we don't need the else I think.

[SjoerdMeijer]

@fhahn : just for completeness, this is following up our our discussion in D105802, and is probably the first of a series of refactoring patches to use more Loop and SCEV here in LoopFlatten.

[RosieSumpter]

• Removed setting inductionPHI to nullptr
• Removed else

[SjoerdMeijer]

Looks like a good bit of refactoring to me.

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

if (!BackBranch || !BackBranch->isConditional()) {
     return false;
   }
   IterationInstructions.insert(BackBranch);
   LLVM_DEBUG(dbgs() << "Found back branch: "; BackBranch->dump());
   bool ContinueOnTrue = L->contains(BackBranch->getSuccessor(0));
 
   // Find the induction PHI. If there is no induction PHI, we can't do the
   // transformation. TODO: could other variables trigger this? Do we have to
   // search for the best one?
-  InductionPHI = nullptr;
-  for (PHINode &PHI : L->getHeader()->phis()) {
-    InductionDescriptor ID;
-    if (InductionDescriptor::isInductionPHI(&PHI, L, SE, ID)) {
-      InductionPHI = &PHI;
-      LLVM_DEBUG(dbgs() << "Found induction PHI: "; InductionPHI->dump());
-      break;
-    }
-  }
+  InductionPHI = L->getInductionVariable(*SE);
   if (!InductionPHI) {
     LLVM_DEBUG(dbgs() << "Could not find induction PHI\n");
     return false;
   }
+  LLVM_DEBUG(dbgs() << "Found induction PHI: "; InductionPHI->dump());
 
   auto IsValidPredicate = [&](ICmpInst::Predicate Pred) {
     if (ContinueOnTrue)
       return Pred == CmpInst::ICMP_NE || Pred == CmpInst::ICMP_ULT;
     else
       return Pred == CmpInst::ICMP_EQ;
   };