Mangle `OpUMulExtended` builtin arguments as unsigned (#3963)
Reverse translation mangled `OpUMulExtended`'s operands with signed
suffixes (e.g. `ll` for i64) instead of unsigned (`mm`), causing
undefined-reference errors for consumers expecting the unsigned name.
AI-assisted: Claude Sonnet 5 (commercial SaaS)
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/468ac916877e634
