Use cast<T> instead of dyn_cast<T>.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205403 91177308-0d34-0410-b5e6-96231b3b80d8
s/llvm::dyn_cast/dyn_cast/

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205404 91177308-0d34-0410-b5e6-96231b3b80d8