pan/mdg: eliminate references to ins->alu.op

pan/mdg: eliminate references to ins->alu.op

In an effort to simplify MIR by not prepacking instructions, this commit
removes references to `ins->alu.op` so that we can later remove the
`ins->alu` field from midgard_instruction.
Every place that was using ins->op was changed to now use the generic
`ins->op` field instead.
We then reconstruct the `alu.op` field right before emission.
This new field is generic and can contain opcodes for ALU, texture or
load/store instructions. It should be used in conjunction with
`ins->type`, just like the current prepacked `op` field.

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>