Use DPP updates instead of DPP movs in subgroup reductions
Use DPP updates instead of DPP movs in subgroup reductions

This matches what we do in subgroup inclusive scans, and it allows the
LLVM GCN DPP combine pass to combine the DPP operations into the ALU
operations, generating much shorter code overall.

googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,googlestadia/llpc,googlestadia/llpc,googlestadia/llpc,GPUOpen-Drivers/llpc