[Analysis] Introduce getStaticBonusApplied (NFC)

InlineCostCallAnalyzer encourages inlining of the last call to the
static function by subtracting LastCallToStaticBonus from Cost.

This patch introduces getStaticBonusApplied to make available the
amount of LastCallToStaticBonus applied.

The intent is to allow the module inliner to determine whether
inlining a given call site is expected to reduce the caller size with
an expression like:

  IC.getCost() + IC.getStaticBonusApplied() < 0

This patch does not add a use of getStaticBonus yet.

Differential Revision: https://reviews.llvm.org/D134373