[ModuleInliner] Add a cost-benefit-based priority

This patch teaches the module inliner a traversal order designed for
the instrumentation FDO (+ThinLTO) scenario.

The new traversal order prioritizes call sites in the following order:

1. Those call sites that are expected to reduce the caller size

2. Those call sites that have gone through the cost-benefit analaysis

3. The remaining call sites

With this fairly simple traversal order, a large internel benchmark
yields performance comparable to the bottom-up inliner -- both in
terms of the execution performance and .text* sizes.

Big thanks goes to Liqiang Tao for the module inliner infrastructure.

I still have hacks outside this patch to prevent excessively long
compilation or .text* size explosion.  I'm trying to come up with
acceptable solutions in near future.

Differential Revision: https://reviews.llvm.org/D134376