intel/nir: do not apply the fsin and fcos trig workarounds for consts

If we have fsin or fcos trigonometric operations with constant values
as inputs, we will multiply the result by 0.99997 in
brw_nir_apply_trig_workarounds, making the result wrong.

Adjusting the rules so they do not apply to const values we let a
later constant fold to deal with it.

v2:
- Do not early constant fold but only apply the trig workaround for
  non constants (Caio).
- Add fixes tag to commit log (Caio).

Fixes: bfd17c76 "i965: Port INTEL_PRECISE_TRIG=1 to NIR."
Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
(cherry picked from commit 3c474f85)