R600: Add helper functions.

Extract these from some of my other patches, since this
is the only thing really making them dependent on each other.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210627 91177308-0d34-0410-b5e6-96231b3b80d8
Use cast instead of assert + dyn_cast

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210628 91177308-0d34-0410-b5e6-96231b3b80d8