nir/lower_idiv: Convert to lower_instructions

nir/lower_idiv: Convert to lower_instructions

Helps deduplicate some code between the two lowering paths. In
particular, it ports the missing 32-bit? check to the precise pass. This
does not change anything immediately: drivers depending on this to lower
16-bit did not work before due to type mismatches and will not work now
since it'll refuse to lower. But that means sub-32-bit idiv can be
lowered more efficiently in an algebraic pass.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8339>