Those were renamed/merged some time ago but it turns out that
ppir_op_undef can't be shared.
It was being used for undefined ssa operations and for read-before-write
operations that may happen to e.g. uninitialized registers (non-ssa)
inside a loop.
We really don't want to reserve a register for the undef ssa case, but
we must reserve and allocate register for the unitialized register case
because when it happens inside a loop it may need to hold its value
across iterations.

This dummy node might be eliminated with a code refactor in ppir in case
we are able to emit the write and allocate the ppir_reg before we emit
the read. But a major refactor we need this to keep this code to avoid
apparent regressions with the new liveness analysis implementation.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Part-of: <mesa/mesa!3502>