nir/lower_idiv: Use ilt instead of bit twiddling

nir/lower_idiv: Use ilt instead of bit twiddling

The previous code was creating a boolean by doing an arithmetic right-
shift by 31 which produces a boolean which is true if the argument is
negative.  This is the same as the expression r < 0 which is much
simpler and doesn't depend on NIR's representation of booleans.

Reviewed-by: Eric Anholt <eric@anholt.net>