nine: use separate register for aL emulation

nine: use separate register for aL emulation

NIR loop unrolling is only working if the loop counter is a scalar.
So keep the loop counter separate and move the aL emulation and
the aL increment to a new register.

This allows loop unrolling with vec4 backends where unconditional
scalarizing of phi nodes is undesirable, like for example r300.

Signed-off-by: Pavel Ondračka <pavel.ondracka@gmail.com>
Reviewed-by: Axel Davy <davyaxel0@gmail.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7222
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21243>