[LoopFuse] Change DT to reference in FusionCandidate struct. NFC

Assertion added in f50821c confirms that the DT is indeed nonnull.
Change it to a reference instead of a pointer to make this explicit in
FusionCandidate.
Suggested in D118472.

llvm/lib/Transforms/Scalar/LoopFuse.cpp

@@ -178,12 +178,12 @@ struct FusionCandidate {
   /// FusionCandidateCompare function, required by FusionCandidateSet to
   /// determine where the FusionCandidate should be inserted into the set. These
   /// are used to establish ordering of the FusionCandidates based on dominance.
-  const DominatorTree *DT;
+  DominatorTree &DT;
   const PostDominatorTree *PDT;
 
   OptimizationRemarkEmitter &ORE;
 
-  FusionCandidate(Loop *L, const DominatorTree *DT,
+  FusionCandidate(Loop *L, DominatorTree &DT,
                   const PostDominatorTree *PDT, OptimizationRemarkEmitter &ORE,
                   TTI::PeelingPreferences PP)
       : Preheader(L->getLoopPreheader()), Header(L->getHeader()),
@@ -192,7 +192,6 @@ struct FusionCandidate {
         GuardBranch(L->getLoopGuardBranch()), PP(PP), AbleToPeel(canPeel(L)),
         Peeled(false), DT(DT), PDT(PDT), ORE(ORE) {
 
-    assert(DT && "Expected non-null DT!");
     // Walk over all blocks in the loop and check for conditions that may
     // prevent fusion. For each block, walk over all instructions and collect
     // the memory reads and writes If any instructions that prevent fusion are
@@ -391,7 +390,7 @@ struct FusionCandidateCompare {
   /// IF RHS dominates LHS and LHS post-dominates RHS, return false;
   bool operator()(const FusionCandidate &LHS,
                   const FusionCandidate &RHS) const {
-    const DominatorTree *DT = LHS.DT;
+    const DominatorTree *DT = &(LHS.DT);
 
     BasicBlock *LHSEntryBlock = LHS.getEntryBlock();
     BasicBlock *RHSEntryBlock = RHS.getEntryBlock();
@@ -646,7 +645,7 @@ struct LoopFuser {
     for (Loop *L : LV) {
       TTI::PeelingPreferences PP =
           gatherPeelingPreferences(L, SE, TTI, None, None);
-      FusionCandidate CurrCand(L, &DT, &PDT, ORE, PP);
+      FusionCandidate CurrCand(L, DT, &PDT, ORE, PP);
       if (!CurrCand.isEligibleForFusion(SE))
         continue;
 
@@ -991,7 +990,7 @@ struct LoopFuser {
                                                FuseCounter);
 
           FusionCandidate FusedCand(
-              performFusion((Peel ? FC0Copy : *FC0), *FC1), &DT, &PDT, ORE,
+              performFusion((Peel ? FC0Copy : *FC0), *FC1), DT, &PDT, ORE,
               FC0Copy.PP);
           FusedCand.verify();
           assert(FusedCand.isEligibleForFusion(SE) &&