This enables use of MemorySSA instead of MemDep in MemCpyOpt. To allow this without significant compile-time impact, the MemCpyOpt pass is moved directly before DSE (in the cases where this was not already the case) in order to avoid a duplicate MemorySSA construction.
With that done, this change is compile-time positive: http://llvm-compile-time-tracker.com/compare.php?from=f6f6f6375d1a4bced8a6e79a78726ab32b8dd879&to=3bef22406a62ae83712557038305cd135d1b1851&stat=instructions
Run-time impact is to be determined...