[MemorySSA] Add convenience APIs in updater to avoid needing MSSA.

[asbirlea]

Summary

Ideally passes should not need to pass MSSA around and do all updates through the updater.
Add convenience APIs to help with that.

[asbirlea]

Rush edit.

[george.burgess.iv]

LGTM after two nits are addressed. Thank you!

include/llvm/Analysis/MemorySSAUpdater.h

144

nit: const Instruction *. (const MemoryAccess * above might/might not make sense, but here it seems trivially correct)

150

As discussed offline, it probably makes the most sense here to just make make a getMemorySSA(), rather than having dumpMemorySSA()/verifyMemorySSA()/... wrappers

(Whether getMemorySSA returns a const MemorySSA * or MemorySSA * is up to you; I'm happy either way).

[asbirlea]

Thank you for the review!

[asbirlea]

Address comments.

include/llvm/Analysis/MemorySSAUpdater.h

public:
   /// Remove a MemoryAccess from MemorySSA, including updating all
   /// definitions and uses.
   /// This should be called when a memory instruction that has a MemoryAccess
   /// associated with it is erased from the program.  For example, if a store or
   /// load is simply erased (not replaced), removeMemoryAccess should be called
   /// on the MemoryAccess for that store/load.
   void removeMemoryAccess(MemoryAccess *);
 
+  /// Remove MemoryAccess for a given instruction, if a MemoryAccess exists.
+  /// This should be called when an instruction (load/store) is deleted from
+  /// the program.
+  void MemorySSAUpdater::removeMemoryAccess(Instruction *I) {
+    if (MemoryAccess *MA = MSSA->getMemoryAccess(I))
+      removeMemoryAccess(MA);
+  }
+
+  /// Verify MemorySSA correctness after an update
+  void verifyMemorySSA() { MSSA->verifyMemorySSA(); }
+
 private:
   // Move What before Where in the MemorySSA IR.
   template <class WhereType>
   void moveTo(MemoryUseOrDef *What, BasicBlock *BB, WhereType Where);
   MemoryAccess *getPreviousDef(MemoryAccess *);
   MemoryAccess *getPreviousDefInBlock(MemoryAccess *);
   MemoryAccess *
   getPreviousDefFromEnd(BasicBlock *,