[llvm][NFC] Factor out cost-model independent inling decision

Summary:
llvm::getInlineCost starts off by determining whether inlining should
happen or not because of user directives or easily determinable
unviability. This CL refactors this functionality as a reusable API.

Reviewers: davidxl, eraman

Reviewed By: davidxl, eraman

Subscribers: hiraditya, haicheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73825