nir/lower_idiv: Be less creative about signs

nir/lower_idiv: Be less creative about signs

I'm sorry to whoever wrote this, but

   (x - (int) (x < 0)) ^ -((int) (x < 0))

is not an acceptable way to write iabs.

Shader-db results on Intel Tiger Lake with lower_idiv enabled:

    total instructions in shared programs: 21122548 -> 21122570 (<.01%)
    instructions in affected programs: 2369 -> 2391 (0.93%)
    helped: 2
    HURT: 8

    total cycles in shared programs: 791609360 -> 791608062 (<.01%)
    cycles in affected programs: 114106 -> 112808 (-1.14%)
    helped: 9
    HURT: 1

If we make the Intel back-end less stupid, we get to 9/1 helped/HURT for
instructions as well but that's for a different MR.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17845>