fix(licm): Use Loop::header in Loop::is_fully_executed

[aakoshh]

Description

Problem*

Resolves a problem I noticed.

Summary*

Fixed Loop::is_fully_executed to use self.header instead of let Some(header) = self.blocks.first(). The ID of blocks shouldn't be relied on for which block is the header, especially since we already know exactly since it identifies the Loop. Indeed if we swap the order of blocks, the SSA parser assigns IDs in the order of appearance, breaking the assumption made by the method.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: b811ecc	Previous: 9773b54	Ratio
test_report_zkpassport_noir-ecdsa_	3 s	2 s	1.50
test_report_zkpassport_noir_rsa_	2 s	1 s	2

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Execution Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: bb9e57d	Previous: 9773b54	Ratio
private-kernel-inner	0.021 s	0.017 s	1.24
rollup-root	0.005 s	0.004 s	1.25

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench