InputCommon/ExpressionParser: Function argument parsing minor cleanup.

members instead of passing arguments.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

The general convention in the codebase is to compare the non-constant
value/string with the constant value/string, not the other way around.

Given all conditional bodies only contain a return, the use of else here
isn't necessary.

This has the benefit of consistently vertically aligning the names.