[ConstraintElim] Refactor GEP offset collection.

Move GEP offset collection to separate helper function and collect
variable and constant offsets in OffsetResult. For now, this only
supports 1 VariableOffset, but the new code structure can be more easily
extended to handle more offsets in the future.

The refactoring drops the check that the VariableOffset >= -1 * constant
offset. This is not needed to check whether the constraint is
monotonically increasing. The constant factors can be ignored, the
constraint will be monotonically increasing if all variables are
positive.

See https://alive2.llvm.org/ce/z/ah2uSQ,
    https://alive2.llvm.org/ce/z/NCADNZ