[IROutliner] Adding a cost model, and debug option to turn the model off.
This adds a cost model that takes into account the total number of
machine instructions to be removed from each region, the number of
instructions added by adding a new function with a set of instructions,
and the instructions added by handling arguments.
Tests not adding flags:
Reviewers: jroelofs, paquette
Differential Revision: https://reviews.llvm.org/D87299