VOP3b instructions like v_addc_u32 write vcc (an sgpr) as well as a vgpr
result. The way this was modelled made the write to vcc take an extra
micro-op, which made the whole instruction take twice as long to issue,
which is inaccurate.
Fix this by introducing a new write class that doesn't consume any
resources.
The name suggests it is any VCC write, which is not so. In addition it is not always a VCC. Maybe change to WriteAuxSGPR?