nir: Mark negative re-distribution on fadd as imprecise

Otherwise, it would mutate `fneg(fadd(-0, 0))` into `fadd(0, -0)` which
isn't correct since -0 + (+0) = +0 + (-0) = +0.

This fixes the OpenCL contraction tests on Iris.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <!16041>
175 jobs for !16041 with nir/fadd-fneg-redist in 40 minutes and 1 second (queued for 13 seconds)
latest merge request