[MustExecute] Silence clang warning about unused captured 'this'

New code introduced in fe799c97 caused clang to complain with

../lib/Analysis/MustExecute.cpp:360:34: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
  GetterTy<LoopInfo> LIGetter = [this](const Function &F) {
                                 ^~~~
../lib/Analysis/MustExecute.cpp:365:44: error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
  GetterTy<PostDominatorTree> PDTGetter = [this](const Function &F) {
                                           ^~~~
2 errors generated.