[IROutliner] Adding a cost model, and debug option to turn the model off.

[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:

llvm/test/Transforms/IROutliner/outlining-cost-model.ll

Reviewers: jroelofs, paquette

Differential Revision: https://reviews.llvm.org/D87299