Fix side effects in if-let expressions

[ironcev]

Description

This PR fixes the critical issue in if-lets: #5173

if-lets were desugared to match expressions but used their own implementation for desugaring which wasn't storing the result of the matched value into a compiler generated variable but instead re-evaluated it on every access, causing side effects.

The new implementation generates exactly the same desugaring as the equivalent match expression would have, simply by forwarding the if-let desugaring to match desugaring.

Since the generated IR is different, the IR gen tests needed to be adapted as well.
Also, a single assert is commented out in the go_to_definition_for_matches LSP test until #5221 is solved.

Closes #5173.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
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