IR tests snapshots

[xunilrj]

Description

This PR is a necessary change for #7646.

Some IR tests have a bug: their harness is expecting some passes to return modified = true even when the pass is not modifying anything. To make sure that #7646 is NOT changing the optimisation itself, but only fixing the modified flag, I am creating this PR.

The idea is that #7646 snapshot tests will have only the flag changed, not their optimisation.

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.
• I have requested a review from the relevant team or maintainers.

[cursor]

PR Summary

Low Risk
Test harness and snapshot files only; compiler optimization logic is not changed in this diff.

Overview
Adds insta snapshot coverage for sway-ir optimization pass tests so each .ir fixture records the pass’s Modified boolean plus a unified before/after IR diff (with ANSI stripped via vte).

The harness in tests.rs now runs only *.ir files, builds that diff with prettydiff, writes *.ir.snap next to the test, and still enforces FileCheck on the optimized IR. A large set of new snapshots under sway-ir/tests/** captures current pass behavior (DCE, inline, mem2reg, demote_*, etc.) as a baseline ahead of fixing incorrect modified reporting.

Minor tooling tweaks: perf-snapshot-historical is enabled on macOS in the justfile, and a trailing newline is fixed on test-forc-fmt-check-panic.

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