VecUtils: Clean up uses of dyn_cast.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179936 91177308-0d34-0410-b5e6-96231b3b80d8
Don't litter .s files in test directory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179937 91177308-0d34-0410-b5e6-96231b3b80d8