NFC: [LICM] Update some stale comments

[djtodoro]

Summary

After removing the MaybePromotable, some comments became stale. This improves them.

[nikic]

LGTM

llvm/lib/Transforms/Scalar/LICM.cpp

if (!HasCatchSwitch) {
       for (BasicBlock *ExitBlock : ExitBlocks) {
         InsertPts.push_back(&*ExitBlock->getFirstInsertionPt());
         MSSAInsertPts.push_back(nullptr);
       }
 
       PredIteratorCache PIC;
 
       // Promoting one set of accesses may make the pointers for another set
-      // loop invariant, so run this in a loop (with the MaybePromotable set
-      // decreasing in size over time).
+      // loop invariant, so run this in a loop.
       bool Promoted = false;
       bool LocalPromoted;
       do {
         LocalPromoted = false;
         for (const SmallSetVector<Value *, 8> &PointerMustAliases :
              collectPromotionCandidates(MSSA, AA, L)) {
           LocalPromoted |= promoteLoopAccessesToScalars(
               PointerMustAliases, ExitBlocks, InsertPts, MSSAInsertPts, PIC, LI,
collectPromotionCandidates(MemorySSA *MSSA, AliasAnalysis *AA, Loop *L) {
   auto IsPotentiallyPromotable = [L](const Instruction *I) {
     if (const auto *SI = dyn_cast<StoreInst>(I))
       return L->isLoopInvariant(SI->getPointerOperand());
     if (const auto *LI = dyn_cast<LoadInst>(I))
       return L->isLoopInvariant(LI->getPointerOperand());
     return false;
   };
 
-  // Populate AST with potentially promotable accesses and remove them from
-  // MaybePromotable, so they will not be checked again on the next iteration.
+  // Populate AST with potentially promotable accesses.
   SmallPtrSet<Value *, 16> AttemptingPromotion;
   foreachMemoryAccess(MSSA, L, [&](Instruction *I) {
     if (IsPotentiallyPromotable(I)) {
       AttemptingPromotion.insert(I);
       AST.add(I);
     }
   });