build: preserve DebugRefs during SSA order repairs

[cpunion]

Problem

LLGo repairs Go SSA evaluation order in two places: return values whose storage is mutated by a later call, and assignment targets for single-case select receives. With ssa.GlobalDebug, ssa.DebugRef metadata also uses the affected values. Treating those metadata uses as executable dependencies blocks both otherwise-safe repairs.

That makes enabling DWARF change program behavior:

• crypto/x509.ParseOID can return a stale OID because its result load stays before the mutating call.
• A single-case select can evaluate its assignment target before the receive; the acceptance test observes [100 1] instead of [1 100].

Changes

• move a repaired return load and its ssa.DebugRef users as one stable group after the mutating call
• move ssa.DebugRef users together with single-case select assignment-target dependencies after the receive
• continue rejecting either transformation when an executable instruction would observe a moved value too early

The change is confined to the existing SSA order-repair module. It does not alter the loader, DWARF encoding, C ABI lowering, or LLVM optimization pipeline.

Fixes #2122.

Verification

• synthetic pointer, map, and two-value select regressions with default SSA and ssa.GlobalDebug
• plain receive and multi-case select non-regressions
• return-order regressions with default SSA and ssa.GlobalDebug
• direct crypto/x509.ParseOID SSA regression
• exact TestSingleCaseSelectReceiveAssignOrder acceptance with -w=false on macOS and Ubuntu arm64
• crypto/x509 TestOID with -w=false on macOS and Ubuntu arm64
• full internal/build package on macOS
• 100% statement coverage for the new select DebugRef grouping helper

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[codecov]

Codecov Report

❌ Patch coverage is 96.62921% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/compile.go	85.24%	6 Missing and 3 partials ⚠️
cl/debug_scope.go	90.90%	2 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[cpunion]

Consolidated into #2143, which now contains the SSA DebugRef order-repair changes and their regression coverage. Closing this redundant PR; no code is being merged to main by this action.