swift234

Add an LLVM pass to merge similar functions.
It's like LLVM's MergeFunctions pass, except that it can also merge functions which differ by some constants.
The intention is to merge specialized functions which only differ by metadata lookups. But it can also merge other types of functions.
It gives ~7% code size reducation for the stdlib.

There are still some open TODOs, e.g. to share common code with LLVM's MergeFunctions pass (currently much code is just copied).