nv50/ir/ra: improve condition for short regs, unify with cond for 16-bit

nv50/ir/ra: improve condition for short regs, unify with cond for 16-bit

Instead of the size restriction existing in two places, and potentially
being applied twice, we move this together. Ops with 16-bit register
addresses can only take a short reg, and ops with immediates can only
take a short reg.

Of course we leave the immediate 0 in place since we know that it will
be replaced by r63/r127 down the line, so don't treat zeroes as an
immediate.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Karol Herbst <kherbst@redhat.com>