[InlineCost] Make cost-benefit decision explicit

With cost-benefit analysis for inlining, we bypass the cost-threshold by returning inline result from call analyzer early.

However the cost and threshold are still available from call analyzer, and when cost is actually higher than threshold, we incorrect set the reason.

The change makes the decision from cost-benefit analysis explicit. It's mostly NFC, except that it allows the priority-based sample loader inliner used by CSSPGO to use cost-benefit heuristic.

Differential Revision: https://reviews.llvm.org/D99302