[zhouguangyuan0718]

LTO package compilation currently serializes prepared but unoptimized IR. Run LLVM's lto-pre-link<O2> pipeline before writing the package bitcode, leaving whole-program lto<O2> and late pre-codegen lowering to the linker. Ordinary object compilation keeps its existing O0/O2 selection.

The change reuses the compiler's existing optimization path. With -llvm-keep-ir, .raw.ll remains the frontend snapshot, .ll remains prepared IR, and .opt.ll now records the pre-link optimized LTO input. Verify the optimized module before serialization. Existing unsupported-flag checks remain unchanged.

The amd64/arm64 archive regression disables inlining and verifies that LLVM constant propagation removes a pure package-local call before bitcode serialization, while both definitions retain their original package symbol indices and the archive's export data remains usable. This regression fails against #355 before the change. Focused emission, early IR, optimization-mode and flag-rejection tests pass; ordinary CPU/FMV runtime behavior also passes.

End-to-end validation uses a separate checkout combining this patch with #347; the production PR remains based directly on the merged #355 commit 8053408d154c. The FullLTO CPU/FMV runtime and complete integration suite pass (265.422s), including content hashing, assembly, cache reuse, GC/stack growth, reflection, traceback, logical DWARF packages, linkname positive/negative cases, mixed ordinary objects, PIE and cgo automatic/external linking.

Performance qualification is still open. One macOS arm64 sample of the 45-package runtime fixture, with package caches warmed and the link forced fresh, gave:

Measurement	Prepared IR only	Pre-link O2
Input bitcode	16,095,200 B	18,193,840 B
Combined optimized bitcode	16,570,944 B	17,744,944 B
Executable	2,676,642 B	2,741,842 B
Warm-cache build wall time	16.39 s	17.05 s
Maximum RSS reported by time	1,531,494,400 B	1,567,326,208 B

This is a single sample, not a statistical benchmark or a measured general speedup. The size increase is a reason to keep this PR in draft for review. There are no custom LLVM pass lists or relaxed Go symbol-identity checks.

Builds on #355; companion to #347. Do not merge automatically.