[Polly] Fix codegen assertions to account for DefinedBehaviorContext (#209188)

DeLICM may produce new read access relations whose domain is restricted
to the DefinedBehaviorContext (e.g., only valid when a parameter ensures
no UB). The validation in setNewAccessRelation already accounts for
this, but the debug assertions in createNewAccesses and
generateScalarLoads did not, causing false assertion failures during
code generation.

Intersect the checked domains with getBestKnownDefinedBehaviorContext()
to match the contract that DeLICM relies on.

Fixes #205732

GitOrigin-RevId: ea612545644d1e3f238ac4cd053758f569f9bf3b