Refactor code-size reduction estimation methods out of InlineCostAnalyzer and...

Refactor code-size reduction estimation methods out of InlineCostAnalyzer and into CodeMetrics.  They
don't use any InlineCostAnalyzer state, and are useful for other clients who don't necessarily want to use
all of InlineCostAnalyzer's logic, some of which is fairly inlining-specific.

No intended functionality change.

llvm-svn: 113499