lima/ppir: handle write to dead registers in ppir

nir can output writes to dead registers when expanding vec4 operations
to non-ssa registers. In that case, some components of the vec4 may be
assigned but never read. The ppir scheduler reorders instructions and
may place such an instruction writing to a dead register somewhere else
in the program.
In order to prevent regalloc from allocating a live register for this
operation, an interference must be assigned to it during liveness
analysis.

This workaround may be removed in the future if the assignments to dead
components can be removed earlier in ppir or nir.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>