refactor: address PTO IR codecheck findings

[TaoTao-real]

Summary

• Split lib/PTO/IR/PTO.cpp into 14 codecheck-sized include fragments while preserving one translation unit and declaration order.
• Fix the severity-1 division guards and remove side-effecting assertions reported by the scanner.
• Keep all includes before namespace aliases/usings.
• Refactor parsePTOTypeAllowNoBang and TDivSOp::parse into focused helpers to reduce method size and cyclomatic complexity without dropping LLVM 19 pointer address-space syntax.

Baseline

• Based on the current LLVM 19 main.
• Mergeable with main; no conflict at the current head.
• Related GitCode MR: https://gitcode.com/cann/pto-as/merge_requests/178

Testing

• git diff --check — passed
• LLVM 19.1.7 ninja -C build PTOIR — passed
• ninja -C build check-pto reaches an unrelated local tool build failure because the stock LLVM 19 checkout does not define Bisheng-specific llvm::CallingConv::SimtEntry; GitHub Actions remains the authoritative full gate.

Scanner status

The attached unresolved export was reviewed. The severity-1 findings are addressed. Structural findings are being removed through method-level helper extraction; an external CleanCode rescan is still required because the scanner is not part of this repository.

Review focus

• Verify the single-translation-unit fragment order.
• Verify parser helper extraction preserves accepted syntax and operand order.
• Verify explicit division guards preserve dynamic/error behavior.