The MemorySSA-based implementation has been enabled for a few months (since D94376) without any issues that I'm aware of. 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.
clang-tidy: warning: 'auto *MD' can be declared as 'const auto *MD' [llvm-qualified-auto]
not useful