Fix sphinx warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218081 91177308-0d34-0410-b5e6-96231b3b80d8
Use cast<> instead of unchecked dyn_cast<>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218085 91177308-0d34-0410-b5e6-96231b3b80d8