[ConstraintElim] Move just before loop simplification pipeline.

Adjust the pipeline slightly to move ConstraintElim just before the loop
simplification pipeline. This increases the number of cases where SCEV
should can preserved in the future.

This also enables slightly more opportunities, by benefiting from
earlier CFG simplifications, which allow more conditions to be added.

Reviewed By: nikic, antoniofrighetto

Differential Revision: https://reviews.llvm.org/D158843