Verify that entry block does not have predecessors

[ironcev]

Description

This PR fixes #6148 by verifying that a function entry block cannot have predecessors.

Additionally, the PR provides a simple testing infrastructure for testing IR verifications.

Closes #6148.

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).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• 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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