As per my discussions with Anna, removed MayWriteAndHaveSideEffects.

The reason we decided to do this is that MayHaveSideEffects implies that a write
can occur, making MayWriteAndHaveSideEffects redundent.

Swift SVN r7943

include/swift/SIL/SILInstruction.h

@@ -52,8 +52,7 @@ enum class SILInstructionMemoryBehavior {
   ///        result/users represent its effect.
   MayHaveSideEffects,
   /// \brief The instruction may write to memory.
-  MayWrite,
-  MayWriteAndHaveSideEffects
+  MayWrite
 };
 
 enum IsTake_t { IsNotTake, IsTake };