nir: Fix nir_opt_idiv_const when negatives are involved

First, allow the case for negative powers of two.  Then ensure that we
use the absolute value of the non-constant value to calculate the
quotient -- this was hinted in the code by the name 'uq'.

This fixes an issue when 'd' is positive and 'n' is negative.  The
ishr will propagate the negative sign and we'll use nir_ineg() again,
incorrectly.

v2: First version used only ishr, but that isn't sufficient, since it
    never can produce a zero as a result.  (Jason)
    Allow negative powers of two.  (Caio)

Fixes: 74492eba "nir: Add a pass for lowering integer division by constants"
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 8a995f2b)