[llvm][NFC] Style fixes in Inliner.cpp
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
Differential Revision: https://reviews.llvm.org/D77837