Reduce expression complexity to appease compiler (Xcode9b5)

Ref: #467 #470

Sources/CryptoSwift/AES.swift

@@ -322,19 +322,14 @@ fileprivate extension AES {
         var rk2: Array<Array<UInt32>> = expandKey(key, variant: variant)
 
         for r in 1 ..< rounds {
-            var w: UInt32
-
-            w = rk2[r][0]
-            rk2[r][0] = U1[Int(B0(w))] ^ U2[Int(B1(w))] ^ U3[Int(B2(w))] ^ U4[Int(B3(w))]
-
-            w = rk2[r][1]
-            rk2[r][1] = U1[Int(B0(w))] ^ U2[Int(B1(w))] ^ U3[Int(B2(w))] ^ U4[Int(B3(w))]
-
-            w = rk2[r][2]
-            rk2[r][2] = U1[Int(B0(w))] ^ U2[Int(B1(w))] ^ U3[Int(B2(w))] ^ U4[Int(B3(w))]
-
-            w = rk2[r][3]
-            rk2[r][3] = U1[Int(B0(w))] ^ U2[Int(B1(w))] ^ U3[Int(B2(w))] ^ U4[Int(B3(w))]
+            for i in 0..<4 {
+                let w = rk2[r][i]
+                let u1 = U1[Int(B0(w))]
+                let u2 = U2[Int(B1(w))]
+                let u3 = U3[Int(B2(w))]
+                let u4 = U4[Int(B3(w))]
+                rk2[r][i] = u1^u2^u3^u4
+            }
         }
 
         return rk2