improve iplist comments. Switch iplist from allocating its sentinal object (for

improve iplist comments.  Switch iplist from allocating its sentinal object (for
end()) eagerly to allocating it lazily.  This saves a lot of memory for JIT applications
that read a module but don't materialize most of the functions (e.g. 62K for 252.eon).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@41142 91177308-0d34-0410-b5e6-96231b3b80d8