[PowerPC] return early if there is no preparing candidate in the loop; NFC

[shchenz]

Summary

This is from @jsji review comments for patch D108750.

This is to improve some compiling time.

[jsji]

LGTM.

llvm/lib/Target/PowerPC/PPCLoopInstrFormPrep.cpp

namespace {
 
   private:
     PPCTargetMachine *TM = nullptr;
     const PPCSubtarget *ST;
     DominatorTree *DT;
     LoopInfo *LI;
     ScalarEvolution *SE;
     bool PreserveLCSSA;
+    bool HasCandidateForPrepare;
 
     /// Successful preparation number for Update/DS/DQ form in all inner most
     /// loops. One successful preparation will put one common base out of loop,
     /// this may leads to register presure like LICM does.
     /// Make sure total preparation number can be controlled by option.
     unsigned SuccPrepCount;
 
     bool runOnLoop(Loop *L);
for (auto &J : *BB) {
       if (L->isLoopInvariant(PtrValue))
         continue;
 
       const SCEV *LSCEV = SE->getSCEVAtScope(PtrValue, L);
       const SCEVAddRecExpr *LARSCEV = dyn_cast<SCEVAddRecExpr>(LSCEV);
       if (!LARSCEV || LARSCEV->getLoop() != L)
         continue;
 
+      // Mark that we have candidates for preparing.
+      HasCandidateForPrepare = true;
+
       if (isValidCandidate(&J, PtrValue, PointerElementType))
         addOneCandidate(&J, LSCEV, Buckets, MaxCandidateNum);
     }
   return Buckets;
 }
 
 bool PPCLoopInstrFormPrep::prepareBaseForDispFormChain(Bucket &BucketChain,
                                                     InstrForm Form) {
auto isDQFormCandidate = [&](const Instruction *I, const Value *PtrValue,
     auto *II = dyn_cast<IntrinsicInst>(I);
     if (II)
       return II->getIntrinsicID() == Intrinsic::ppc_vsx_lxvp ||
              II->getIntrinsicID() == Intrinsic::ppc_vsx_stxvp;
     // Check if it is a P9 vector load/store.
     return ST && ST->hasP9Vector() && (PointerElementType->isVectorTy());
   };
 
+  HasCandidateForPrepare = false;
+
   // Collect buckets of comparable addresses used by loads and stores for update
   // form.
   SmallVector<Bucket, 16> UpdateFormBuckets =
       collectCandidates(L, isUpdateFormCandidate, MaxVarsUpdateForm);
 
   // Prepare for update form.
   if (!UpdateFormBuckets.empty())
     MadeChange |= updateFormPrep(L, UpdateFormBuckets);
+  else if (!HasCandidateForPrepare)
+    // If no candidate for preparing, return early.
+    return MadeChange;
 
   // Collect buckets of comparable addresses used by loads and stores for DS
   // form.
   SmallVector<Bucket, 16> DSFormBuckets =
       collectCandidates(L, isDSFormCandidate, MaxVarsDSForm);
 
   // Prepare for DS form.
   if (!DSFormBuckets.empty())