[IROutliner] Changing outliner to prioritize reductions on assembly rather than IR instruction

Currently, the IROutliner uses a simple metric to outline the largest amount
of IR possible to outline first if it fits the cost model. This is model
loses out on smaller blocks of code that have higher reductions in cost that
are contained within larger blocks of IR.

This reverses the order, where we calculate all of the costs first, and then
reorder and extract items based on the calculated results.

Reviewers: paquette

Differential Revision: https://reviews.llvm.org/D106440