This tool is part of the bugpoint redesign proposal, once finished it will be merged into bugpoint itself, although no specifics have been defined (i.e. could be integrated into the crash debugger or as a sub-tool).
The tool accepts 2 arguments: an interesting-ness test and an IR file to reduce. Once it parses and verifies the input file, it runs multiple Passes to minimize the IR which in turn call the Delta Debugging implementation (Note: an overview of this algorithm can be found in Delta.h).
Each pass must implement a helper function (extractChunksFromModule) which the Delta implementation will run on each iteration. For example, in the ReduceFunctions pass it will modify the module so it only contains chunk-functions, e.g. if the algorithm wants to test if the first 5 functions and the last 2 are interesting, this helper will modify the module so it only contains said functions.
You can also follow the projects progress through the llvm-dev mailing list (the most recent thread can be found here)