LLVM: enable sancov pass partially

It's useful to have TraceCmp based on the results of LLVM optimizations,
while the code coverage bits were emitted by Zig manually, allowing more
careful correlation to points of interest in the source code.

This re-enables the sancov pass in `-ffuzz` mode, but only TraceCmp.

Notably, IndirectCalls is off, which needs to be implemented manually in
the LLVM backend, and StackDepth remains off, because it is not used by
libfuzzer or AFL either.

If stack depth is re-introduced, it can be done with better performance
characteristics by being function call graph aware, and only lowered in
call graph cycles, where its heuristic properties come in useful.

Fixes the fuzzing regression.

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