[NFC] small rename of private member in InlineCost.cpp

Summary:
Follow-up from https://reviews.llvm.org/D71733. Also moved an
initialization to the base class, where it belonged in the first place.

Reviewers: eraman, davidxl

Reviewed By: davidxl

Subscribers: hiraditya, haicheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72949