Add a flag to the FunctionToLoopAdaptor that allows enabling MemorySSA only for the loop pass managers that are known to preserve it.
If an LPM is known to have only loop transforms that *all* preserve MemorySSA, then use MemorySSA if EnableMSSALoopDependency is set.
If an LPM has loop passes that do not preserve MemorySSA, then the flag passed is false, regardless of the value of EnableMSSALoopDependency.
When using a custom loop pass pipeline via passes=..., use keyword loop vs loop-mssa to use MemorySSA in that LPM. If a loop that does not preserve MemorySSA is added while using the loop-mssa keyword, that's an error.
Add the new loop-mssa keyword to a few tests where a difference occurs when enabling MemorySSA.