Rename product fusion pass

[1sgtpepper]

Summary

Rename the existing product loop-fusion pass, CLI option, implementation file, and test directory to FuseProductControlFlowPass / llzk-fuse-product-control-flow. This prerequisite PR handles the naming and registration portion of #484; the feature-specific control-flow implementation and full pass description remain in #484.

Related work

Partially addresses #484, following the maintainer-requested split.

Changes

• Rename the pass, CLI option, implementation file, and product-fusion test directory.
• Update generated pass declarations, registration, and existing fixture invocations.
• Update the TableGen summary to describe control-flow fusion while leaving the full pass description change in Fuse matching product scf.if regions #484, as requested.
• Preserve existing fusion behavior and generated FileCheck output; this PR changes names and paths only.

Testing

All required hosted CI checks pass for head c81db5b7d30367db9eaeea65b20eb798dafd1d4d on the checks. The existing product-fusion FileCheck fixtures pass with the renamed pass option, and their generated check blocks are unchanged.

Submission checklist

• If I am an external contributor, this PR has a linked issue marked approved; otherwise, this does not apply.
• I added or updated tests for all relevant behavior, or explained above why tests are not needed.
• I updated the relevant TableGen or other documentation, or explained above why documentation is not needed.
• I added a changelog entry describing user-visible changes.
• I enabled Allow edits from maintainers.

AI assistance

• No AI tools contributed to this PR.
• AI tools contributed to this PR.

Tools used: Codex

How the tools contributed: Implementation and code review.

How I verified the contribution: Final changes reviewed.

[project-llzk-issue-triage]

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Closes #123

[tim-hoffman]

LGTM