bifrost: Add support for nir_op_ishl

bifrost: Add support for nir_op_ishl

Bifrost's bitwise ops include the shift capability. Previously we had
hardcoded the shift to zero in all cases.

There's room in future to emit slightly better code if a shift and a
bitwise operation can be folded together, but not going after that for
now.

This change also removes the separate BI_SHIFT instruction class as
BI_BITWISE can cover both cases.

Signed-off-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6091>