Improve SmallPtrSetImpl::count implementation

Relying on the find method implies a roundtrip to the iterator world, which is
not costless because iterator creation involves a few check to ensure the
iterator is in a valid position (through the SmallPtrSetIteratorImpl::AdvanceIfNotValid
method). It turns out that the result of SmallPtrSetImpl::find_imp is either
valid or the EndPointer, so there's no need to go through that abstraction,
and the compiler cannot guess it.

Differential Revision: https://reviews.llvm.org/D80708