Issue Description:

Currently the graph lowering pass uses the following member field to track if error has occurred:

bool errorOccurred = false;

It must be set at the lowest level of any call stack (e.g.

https://github.com/apple/swift/blob/d9d16ef9fbadcb5f1b0a73e632176121ae0945a5/lib/SILOptimizer/Mandatory/TFLowerGraph.cpp#L266), and at other places we must check for the error (e.g.

https://github.com/apple/swift/blob/d9d16ef9fbadcb5f1b0a73e632176121ae0945a5/lib/SILOptimizer/Mandatory/TFLowerGraph.cpp#L498). If the check statement is missed, graph lowering can incorrectly proceed despite of encoutering errors, leading to bugs in the code.

The code might be cleaner and less error prone if each function call returns a bool indicating whether something has failed (the LLVM/swift convention is to return true on error). This general "design pattern" of error handling could be to return a util::Status type of object, but returning a bool is probably sufficient for now.