test: review post-ABI autogenerated FileCheck snapshots

[zhouguangyuan0718]

Purpose

This draft stacks the changes from #2307, #2308, #2309, #2310, and #2311, then switches the default IR check snapshot to post-target-ABI, pre-optimization IR and refreshes only the 40 litgen-maintained fixtures.

The purpose is to review the shape and maintainability of those 40 generated snapshots before migrating handwritten checks.

Scope

• move the module observation point after aggregate and target ABI lowering, before LLVM optimization
• make litgen generate and merge target snapshots, keeping common lines under CHECK and only differing hunks under target prefixes
• refresh exactly 40 autogenerated fixtures on Darwin/arm64 and Linux/amd64
• remove the obsolete literal out.ll generation path
• do not migrate handwritten FileCheck fixtures in this draft

Expected CI status

This draft is intentionally not merge-ready. Handwritten fixtures still describe the previous IR stage, so the full compiler fixture suite is expected to fail until those checks are analyzed and migrated separately. Infrastructure unit tests and the 40 regenerated snapshots are the review target.