sprinkle some llvm_unreachable for MSVC (NFC)

MSVC does not realize that the switch is exhaustive and requires that
the path is explicitly marked as unreachable.  This silences the C4715
warning ("not all control paths return a value").

lib/SILOptimizer/Analysis/MemoryBehavior.cpp

@@ -165,6 +165,7 @@ public:
 
       return MemBehavior::MayWrite;
     }
+    llvm_unreachable("invalid access kind");
   }
 
   MemBehavior visitEndAccessInst(EndAccessInst *endAccess) {