[DFSan] Avoid replacing uses of functions in comparisions.

This can cause crashes by accidentally optimizing out checks for
extern_weak_func != nullptr, when replaced with a known-not-null wrapper.

This solution isn't perfect (only avoids replacement on specific patterns)
but should address common cases.

Internal reference: b/185245029

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D123701