R600: Remove outdated comment

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219777 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Use existing variable

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219778 91177308-0d34-0410-b5e6-96231b3b80d8