pan/mdg: prepare effective_writemask()

In the next commits we will be removing the `alu` field from
midgard_instruction in order to simplify the code.
effective_writemask() doesn't actually use `alu` for anything, it only
needs to know the opcode.

Signed-off-by: Italo Nicola <italonicola@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5933>
4 files changed