nvir: introduce OP_SHF

nvir: introduce OP_SHF

We already use a hack from NVC0LegalizeSSA::handleShift() on GK110 and
newer which encodes SHF into the existing SHL/SHR opcodes, but there's
a couple of problems with it:

- LO/HI are swapped in one of the directions, which is very confusing.
- The initial SM70 code will emit this from NIR->NVIR, and using the
  existing encodings will confuse the optimisation passes.

As I want to limit the impact on other GPUs from the initial bring-up
of Volta/Turing, let's add an explicit representation of SHF in the IR.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5377>