This is done because the CodeGen library and Passes library both link
against Analysis, to avoid adding a dependency between CodeGen and
Passes if we want to extend the DroppedVariableStats code for MIR stats
as well, as seen in https://github.com/llvm/llvm-project/pull/120501
[NFC] Move DroppedVariableStats to its own file and redesign it to be
extensible. (#115563)
Move DroppedVariableStats code to its own file and change the class to
have an extensible design so that we can use it to add dropped
statistics to MIR passes and the instruction selector.