nv50/ir: add lowering helper

nv50/ir: add lowering helper

if we start supporting multiple input IRs we might want to move lowering code
into a common place and keep the initial translation simplier.

This will also allows us to react on ISA changes more easily.

v5: also handle SAT
v6: rename type variables
    fixed lowering of NEG
    add lowering of NOT
v8: don't require C++11 features

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Pierre Moreau <pierre.morrow@free.fr>