correct const-ness with auto and dyn_cast

1. Use const with autos.
2. Don't bother with explicit const in cast ops because they do it automagically.

Thanks, David B. / Aaron B. / Reid K.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219817 91177308-0d34-0410-b5e6-96231b3b80d8