Simplifying use_iterators by dereferencing

Simplifying use_iterators by dereferencing
is not a good idea. The codebase does not depend
in this any more, and it may introduce hidden
runtime cost. If you get compile errors, please
dereference your iterator before passing to cast<>
(and friends).

Also: please consider caching the result of
operator* and reusing that instead of dereferencing
many times.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109220 91177308-0d34-0410-b5e6-96231b3b80d8