nir: Silence unused parameter warnings in generated nir_constant_expressions code

nir: Silence unused parameter warnings in generated nir_constant_expressions code

Reduces my build from 2075 warnings to 2023 warnings by silencing 52
instances of things like

src/compiler/nir/nir_constant_expressions.c: In function ‘evaluate_bfi’:
src/compiler/nir/nir_constant_expressions.c:1812:61: warning: unused parameter ‘bit_size’ [-Wunused-parameter]
 evaluate_bfi(MAYBE_UNUSED unsigned num_components, unsigned bit_size,
                                                             ^~~~~~~~

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>