build: add link-time dead method elimination

[luoliwoshang]

Summary

• Add package metadata summaries for DCE facts, including ordinary edges, type children, interface method demands, interface conversions, reflection method use, and ABI method slots.
• Persist metadata in the package build cache and merge package-local summaries into a global metadata view for analysis.
• Add a metadata-driven deadcode analyzer that computes live ABI method slots using reachable interface demands, used-in-interface types, reflection rules, and MethodByName rules.
• Emit strong ABI type metadata overrides that clear dead method IFn/TFn references, allowing the linker to drop unused method bodies.
• Add metadata golden tests, metadump tooling, and focused analyzer/DCE override coverage.

Notes

• The temporary verbose cache-hit meta read timing output has been removed; -v cache hit logging now prints only the package path.

Test Plan

• go test ./internal/deadcode ./internal/dcepass ./internal/metadata ./internal/build -count=1

Demo Size Comparison

Built _demo/go demos with DCE from this PR into _demo/go/520-dce, and with local upstream/main (ec1b2f74) into _demo/go/520-nodce.

Build command shape:

• LLGO_BUILD_CACHE=off llgo build -a -o <output> .

Summary for demos that built successfully on both sides:

• Matched demos: 55
• No-DCE total size: 59,670,624 bytes
• DCE total size: 43,519,232 bytes
• Total saved: 16,151,392 bytes
• Total reduction: 27.07%
• Demos that grew: 0

Two demos failed on both sides and were excluded from the size total:

• defer/setjmp: C source files are present but this direct llgo build . path is not using cgo/SWIG.
• defer/test: existing source errors around c.Printf / c.Str.

Top reductions:

Demo	No DCE	DCE	Saved	Saved %
goimporter-1389	4267280	2875648	1391632	32.61%
embedunexport-1598	3056032	1762912	1293120	42.31%
gotypes	3072896	2458912	613984	19.98%
abimethod	1434896	912224	522672	36.43%
mimeheader	1527664	1005856	521808	34.16%
reflectpointerto	1719408	1236656	482752	28.08%
reflectmake	1905136	1443568	461568	24.23%
netip	1504976	1048512	456464	30.33%
gobuild-1389	1590144	1158624	431520	27.14%
randdemo	1415280	985376	429904	30.38%
gotoken	1433408	1003808	429600	29.97%
sysexec	1414528	985376	429152	30.34%
oslookpath	1414144	985008	429136	30.35%
sync	1419840	990800	429040	30.22%
createtemp-1654	1431072	1002992	428080	29.91%

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 76.80328% with 283 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/metadata/decode.go	48.54%	53 Missing and 53 partials ⚠️
internal/dcepass/dcepass.go	59.71%	57 Missing and 28 partials ⚠️
internal/metadata/global_summary.go	77.38%	24 Missing and 21 partials ⚠️
internal/metadata/meta.go	67.56%	14 Missing and 10 partials ⚠️
internal/metadata/builder.go	84.84%	5 Missing and 5 partials ⚠️
internal/deadcode/analyze.go	95.91%	3 Missing and 3 partials ⚠️
internal/metadata/format.go	93.97%	3 Missing and 2 partials ⚠️
internal/metadata/encode.go	98.79%	0 Missing and 1 partial ⚠️
ssa/abitype.go	98.66%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[luoliwoshang]

补充一个 DCE 方法槽修复点：

对于被判定为 dead 的方法槽，IFn/TFn 不应该写成 null。这里需要和 Go linker 的做法一致，重定向到一个专门的 runtime.unreachableMethod。

原因是运行时构造 itab 时，fun[0] == 0 有特殊含义：表示该类型没有实现目标接口。如果 DCE 把匹配到的方法函数指针清成 0，类型方法签名虽然仍然匹配，但生成出来的 itab 会被当成“不完整实现”，不会进入 itab cache。

这会导致同一个 concrete type 到同一个接口的转换生成多个不同 itab。典型表现是 reflect.Type 作为 map key 时出现不一致：

t0 == t1 // true
m[t1]    // false

最终会影响标准库里这类逻辑，例如 testing.F.Add(int) 中的：

supportedTypes[reflect.TypeOf(5)]

所以现在的策略是：dead method slot 仍保留非零函数入口，指向 runtime.unreachableMethod。如果该方法真的在运行时被调用，会像 Go 一样明确 throw；同时 itab 的实现判断和缓存语义不会被破坏。