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

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

In an effort to simplify MIR by not prepacking instructions, this commit
removes references to `ins->alu.reg_mode` so that we can later remove
the `ins->alu` field from midgard_instruction.
Every place that was using reg_mode was changed to now use the generic
`ins->src_type` field instead.
We then reconstruct the reg_mode field right before emission.

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>