[LoopInfo] Faster implementation of setLoopID. NFC.

[loladiro]

Summary

This change was part of D46460. However, in the meantime rL341926 fixed the
correctness issue here. What remained was the performance issue in setLoopID
where it would iterate through all blocks in the loop and their successors,
rather than just the predecessor of the header (the later presumably being
much faster). We already have the getLoopLatches to compute precisely these
basic blocks in an efficient manner, so just use it (as the original commit
did for getLoopID).

[jdoerfert]

LGTM

lib/Analysis/LoopInfo.cpp

 }
 
 void Loop::setLoopID(MDNode *LoopID) const {
   assert((!LoopID || LoopID->getNumOperands() > 0) &&
          "Loop ID needs at least one operand");
   assert((!LoopID || LoopID->getOperand(0) == LoopID) &&
          "Loop ID should refer to itself");
 
-  BasicBlock *H = getHeader();
-  for (BasicBlock *BB : this->blocks()) {
-    Instruction *TI = BB->getTerminator();
-    for (BasicBlock *Successor : successors(TI)) {
-      if (Successor == H) {
-        TI->setMetadata(LLVMContext::MD_loop, LoopID);
-        break;
-      }
-    }
-  }
+  SmallVector<BasicBlock *, 4> LoopLatches;
+  getLoopLatches(LoopLatches);
+  for (BasicBlock *BB : LoopLatches)
+      BB->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopID);
 }
 
 void Loop::setLoopAlreadyUnrolled() {
   LLVMContext &Context = getHeader()->getContext();
 
   MDNode *DisableUnrollMD =
       MDNode::get(Context, MDString::get(Context, "llvm.loop.unroll.disable"));
   MDNode *LoopID = getLoopID();