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"