[pytorch] fix code analyzer for LLVM 9 & 10 (#42135)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42135
Tested the code analyzer with LLVM 9 & 10 and fixed a couple issues:
- Rename local demangle() which is available as public API since LLVM 9;
- Fix falsely associated op registrations due to the `phi` instruction;
Test Plan: Imported from OSS
Reviewed By: iseeyuan
Differential Revision: D22795508
Pulled By: ljk53
fbshipit-source-id: 2d47af088acd3312a7ea5fd9361cdccd48940fe6
