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.

Signed-off-by: Alyssa Rosenzweig alyssa@rosenzweig.io Reviewed-by: Jason Ekstrand jason.ekstrand@collabora.com Reviewed-by: Ian Romanick ian.d.romanick@intel.com