⚡ Optimize instruction string building in Canonicalizer

[xkilldash9x]

💡 What:

• Added scratch field (strings.Builder) to Canonicalizer struct to reuse buffer across instructions.
• Refactored processInstruction to use c.scratch instead of a local variable.
• Replaced fmt.Sprintf with c.scratch.WriteString for high-frequency instructions: BinOp, UnOp, Store, If, Jump, Return.
• Updated helper functions (writeSelect, writePhi, writeCallCommon) to accept the shared builder.

🎯 Why:

• Previously, processInstruction allocated a new strings.Builder (and its internal buffer) for every single instruction processed.
• fmt.Sprintf added overhead for parsing format strings and allocating intermediate strings.
• This hot path optimization reduces GC pressure and improves analysis speed.

📊 Measured Improvement:
Benchmark: BenchmarkCanonicalizeFunction (added in pkg/analysis/ir/benchmark_test.go)

• Baseline: 54783 ns/op, 336 allocs/op
• Optimized: 52388 ns/op, 312 allocs/op
• Delta: ~4.4% speedup, 24 fewer allocations per op (~7% reduction).

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PR created automatically by Jules for task 4018986549606110049 started by @xkilldash9x

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