[Polly] Simplify domains before gist.

The compilation of the file
526.blender_r/src/blender/source/blender/editors/space_logic/logic_ops.c
from the SPEC CPU 2017 benchmarks took excessive time to compute

    InvalidDomain.gist_params(Ctx)

Simplifying beforehand, specifically using isl_set_detect_equalities,
reduces the computation time to a negible level again.