[NFC] Refactor loop metadata movemen

[urnathan]

Summary

I happened to touch this code locally and noticed a couple of things:

1. Let's use 'if (T var = expr)' form
2. llvm.loop is one of the fixed metadata ids, let's just use that

Both #1 and #2 are used for elsewhere in the function for these exact accesses.

llvm/lib/Transforms/Utils/Local.cpp

while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
       // We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
       assert(PN->use_empty() && "There shouldn't be any uses here!");
       PN->eraseFromParent();
     }
   }
 
   // If the unconditional branch we replaced contains llvm.loop metadata, we
   // add the metadata to the branch instructions in the predecessors.
-  unsigned LoopMDKind = BB->getContext().getMDKindID("llvm.loop");
-  Instruction *TI = BB->getTerminator();
-  if (TI)
-    if (MDNode *LoopMD = TI->getMetadata(LoopMDKind))
+  if (Instruction *TI = BB->getTerminator())
+    if (MDNode *LoopMD = TI->getMetadata(LLVMContext::MD_loop))
       for (BasicBlock *Pred : predecessors(BB))
-        Pred->getTerminator()->setMetadata(LoopMDKind, LoopMD);
+        Pred->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopMD);
 
   // Everything that jumped to BB now goes to Succ.
   BB->replaceAllUsesWith(Succ);
   if (!Succ->hasName()) Succ->takeName(BB);
 
   // Clear the successor list of BB to match updates applying to DTU later.
   if (BB->getTerminator())
     BB->back().eraseFromParent();