R600/SI: Don't use strings for single characters

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218221 91177308-0d34-0410-b5e6-96231b3b80d8
Use llvm_unreachable instead of assert(!)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218222 91177308-0d34-0410-b5e6-96231b3b80d8
Fix typo

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218223 91177308-0d34-0410-b5e6-96231b3b80d8