nir/algebraic: refactor inexact opcode restrictions

Refactor the code to avoid calling a lot of time to auxiliary functions
when it is not really needed.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>