[llvm][NFC] Style fixes in Inliner.cpp

Summary:
Function names: camel case, lower case first letter.
Variable names: start with upper letter. For iterators that were 'i',
renamed with a descriptive name, as 'I' is 'Instruction&'.

Lambda captures simplification.

Opportunistic boolean return simplification.

Reviewers: davidxl, dblaikie

Subscribers: eraman, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77837