[LoopFuse] Add assertion for non-null DT in fusion candidate

The code paths analyzed (all constructor invocations of fusion
candidate) pass in a non-null DT.
Adding this assert as requested in D118472 before converting this to a
reference argument.

Author: annamthomas

llvm/lib/Transforms/Scalar/LoopFuse.cpp

@@ -192,6 +192,7 @@ 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