[DomTreeUpdater] Use const auto * when iterating over pointers (NFC).

This silences the warning below:

llvm-project/llvm/lib/Analysis/DomTreeUpdater.cpp:510:20: warning: loop variable 'BB' is always a copy because the range of type 'const SmallPtrSet<llvm::BasicBlock *, 8>' does not return a reference [-Wrange-loop-analysis]
  for (const auto &BB : DeletedBBs) {
                   ^
llvm-project/llvm/lib/Analysis/DomTreeUpdater.cpp:510:8: note: use non-reference type 'llvm::BasicBlock *'
  for (const auto &BB : DeletedBBs) {
       ^~~~~~~~~~~~~~~~
1 warning generated.