[NFC] Used mayWriteToMemory in mayHaveSideEffects.

Previously, the implementation of mayWriteToMemory was duplicated within
mayHaveSideEffects.

Author: nate-chandler

lib/SIL/IR/SILInstruction.cpp

@@ -1102,10 +1102,7 @@ bool SILInstruction::mayHaveSideEffects() const {
   if (mayTrap())
     return true;
 
-  MemoryBehavior B = getMemoryBehavior();
-  return B == MemoryBehavior::MayWrite ||
-    B == MemoryBehavior::MayReadWrite ||
-    B == MemoryBehavior::MayHaveSideEffects;
+  return mayWriteToMemory();
 }
 
 bool SILInstruction::mayRelease() const {