R600/SI: Use a register set to -1 for data0 on ds_inc*/ds_dec*

There is not such thing as a 0-data ds instruction, and the data
operand needs to be a vgpr set to something meaningful.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210756 91177308-0d34-0410-b5e6-96231b3b80d8