[MemCpyOpt] Remove MemDepAnalysis-based implementation

The MemorySSA-based implementation has been enabled for a few months
(since D94376). This patch drops the old MDA-based implementation
entirely.

I've kept this to only the basic cleanup of dropping various
conditions -- the code could be further cleaned up now that there
is only one implementation.

Differential Revision: https://reviews.llvm.org/D102113