`this == nullptr` results in undefined behavior

## Issue
‘this’ pointer cannot be null in well-defined C++ code; pointer may be assumed to always convert to true

## Tasks
- [ ] Add compiler warning
clang: [`-Wundefined-bool-conversion`](https://clang.llvm.org/docs/DiagnosticsReference.html#id939)
msvc: ??
- [x] #6847
- [ ] [`IR.cpp`](https://github.com/chakra-core/ChakraCore/blob/2af598f04ab508f9231d6e26f0f82f5a57561413/lib/Backend/IR.cpp#L4852)
- [x] Find further places where this may happen