[SIFoldOperands] Small code cleanups, NFC.

I've been trying to understand the backend better and decided to read the code of this pass.
While doing so, I noticed parts that could be refactored to be a tiny bit clearer.
I tried to keep the changes minimal, a non-exhaustive list of changes is:
- Stylistic changes to better fit LLVM's coding style
- Removing dead/useless functions (e.g. FoldCandidate had getters, but it's a public struct!)
 - Saving regs/opcodes in variables if they're going to be used multiple times in the same condition

Reviewed By: arsenm, foad

Differential Revision: https://reviews.llvm.org/D137539