nir: few missing struct names

nir: few missing struct names

nir.h is a bit inconsistent about 'typedef struct {} nir_foo' vs
'typedef struct nir_foo {} nir_foo'.  But missing struct name tags is
inconvenient when you need a fwd declaration without pulling in all
of nir.

So add missing struct name tag for nir_variable, and a couple other
spots where it would likely be useful.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>