refactor rustc_hir_typeck/src/op.rs

[malezjaa]

Fixes #64297

This is a refactoring PR with no logic or behaviour changes. I tried to improve readability of op.rs, which had a few very large functions using some suboptimal syntax. I broke these up into smaller helpers.

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[Kivooeo]

This is a fairly large PR -- could you help reviewers by adding some context to the description? It would be great to have a summary of what was changed, the key decisions made and the reasoning behind them, and ideally a breakdown into separate commits to make it easier to review piece by piece.

[malezjaa]

Updated the description. There weren't really any key decisions to make since this is a refactor with no behaviour changes. As for separate commits, splitting style changes after the fact would be tedious and I'm not sure it would add much value for reviewers.