nir/serialize: remove up to 3 consecutive equal ALU instruction headers

nir/serialize: remove up to 3 consecutive equal ALU instruction headers

vec4 scalarized ALUs typically have 4 equal instruction headers, so remove
the last 3.

There are no bits left in the ALU header for more flags, so future
extensions of NIR will have to use something like instr_type == 15
to describe more complex ALU instructions.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>