nir/lower_io: Fix the unknown-array-index case in get_deref_align

nir/lower_io: Fix the unknown-array-index case in get_deref_align

The current align_mul calculation in the unknown-array-index
calculation is

    align_mul = MIN3(parent_mul,
                     min_pow2_divisor(parent_offset),
                     min_pow2_divisor(stride))

which is certainly correct if parent_offset > 0.  However, when
parent_offset = 0, min_pow2_divisor(parent_offset) isn't well-defined
and our calculation for it is 1 << -1 which isn't well-defined.  That
said.... it's not actually needed.

The offset to the base of the array is

    array_base = parent_mul * k + parent_offset

for some integer k.  When we throw in an unknown array index i, we get

    elem = parent_mul * k + parent_offset + stride * i.

If we set new_align = MIN2(parent_mul, min_pow2_divisor(stride)), then
both parent_mul and stride are divisible by new_align and

    elem = (parent_mul / new_alig) * new_align * k +
           (stride / new_align) * new_align * i + parent_offset

         = new_align * ((parent_mul / new_alig) * k +
                        (stride / new_align) * i) + parent_offset

so elem = new_align * j + parent_offset where

    j = (parent_mul / new_alig) * k + (stride / new_align) * i.

That's a very long-winded way of saying that we can delete one parameter
from the align_mul calculation and it's still fine. :-)

Fixes: 480329cf8b31 "nir: Add a helper for getting the alignment of a deref"
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Tested-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6628>