[benchmark] Rehouse DistinctClassFieldAccesses

Benchmark DistinctClassFieldAccesses was added in a strange place…
See https://github.com/apple/swift/pull/18892/files#r212038958

Per discussion in in swiftlang#18892 (comment)
I’m moving it to the extremely similar `ArrayInClass`, while fixing its workload size (and loop pattern) to make it relatively comparable.

Author: palimondo

benchmark/single-source/ArrayInClass.swift

@@ -11,12 +11,19 @@
 //===----------------------------------------------------------------------===//
 
 import TestsUtils
-public let ArrayInClass = BenchmarkInfo(
-  name: "ArrayInClass",
-  runFunction: run_ArrayInClass,
-  tags: [.validation, .api, .Array],
-  setUpFunction: { ac = ArrayContainer() },
-  tearDownFunction: { ac = nil })
+public let ArrayInClass = [
+  BenchmarkInfo(
+    name: "ArrayInClass",
+    runFunction: run_ArrayInClass,
+    tags: [.validation, .api, .Array],
+    setUpFunction: { ac = ArrayContainer() },
+    tearDownFunction: { ac = nil }),
+  BenchmarkInfo(name: "DistinctClassFieldAccesses",
+    runFunction: run_DistinctClassFieldAccesses,
+    tags: [.unstable, .api, .Array],
+    setUpFunction: { workload = ClassWithArrs(N: 100_000) },
+    tearDownFunction: { workload = nil }),
+]
 
 var ac: ArrayContainer!
 
@@ -41,3 +48,38 @@ public func run_ArrayInClass(_ N: Int) {
   let a = ac!
   a.runLoop(N)
 }
+
+class ClassWithArrs {
+  var N: Int = 0
+  var A: [Int]
+  var B: [Int]
+
+  init(N: Int) {
+    self.N = N
+
+    A = [Int](repeating: 0, count: N)
+    B = [Int](repeating: 0, count: N)
+  }
+
+  func readArr() {
+    for i in 0..<self.N {
+      guard A[i] == B[i] else { fatalError("") }
+    }
+  }
+
+  func writeArr() {
+    for i in 0..<self.N {
+      A[i] = i
+      B[i] = i
+    }
+  }
+}
+
+var workload: ClassWithArrs!
+
+public func run_DistinctClassFieldAccesses(_ N: Int) {
+  for _ in 1...N {
+    workload.writeArr()
+    workload.readArr()
+  }
+}

benchmark/single-source/ExistentialPerformance.swift

@@ -13,7 +13,6 @@
 import TestsUtils
 
 public let ExistentialPerformance = [
-  BenchmarkInfo(name: "DistinctClassFieldAccesses", runFunction: run_DistinctClassFieldAccesses, tags: [.unstable, .api, .Array]),
   BenchmarkInfo(name: "ExistentialTestArrayConditionalShift_ClassValueBuffer1", runFunction: run_ExistentialTestArrayConditionalShift_ClassValueBuffer1, tags: [.unstable, .api, .Array]),
   BenchmarkInfo(name: "ExistentialTestArrayConditionalShift_ClassValueBuffer2", runFunction: run_ExistentialTestArrayConditionalShift_ClassValueBuffer2, tags: [.unstable, .api, .Array]),
   BenchmarkInfo(name: "ExistentialTestArrayConditionalShift_ClassValueBuffer3", runFunction: run_ExistentialTestArrayConditionalShift_ClassValueBuffer3, tags: [.unstable, .api, .Array]),
@@ -115,44 +114,6 @@ public let ExistentialPerformance = [
   BenchmarkInfo(name: "ExistentialTestTwoMethodCalls_IntValueBuffer4", runFunction: run_ExistentialTestTwoMethodCalls_IntValueBuffer4, tags: [.unstable]),
 ]
 
-class ClassWithArrs {
-    var N: Int = 0
-    var A: [Int]
-    var B: [Int]
-
-    init(N: Int) {
-        self.N = N
-
-        A = [Int](repeating: 0, count: N)
-        B = [Int](repeating: 0, count: N)
-    }
-
-    func readArr() {
-        for i in 0..<self.N {
-            for j in 0..<i {
-				let _ = A[j]
-				let _ = B[j]
-			}
-        }
-    }
-
-    func writeArr() {
-        for i in 0..<self.N {
-    			A[i] = i
-    			B[i] = i
-    		}
-    }
-}
-
-let workload = ClassWithArrs(N: 100)
-
-public func run_DistinctClassFieldAccesses(_ N: Int) {
-    for _ in 1...N {
-        workload.writeArr()
-        workload.readArr()
-    }
-}
-
 protocol Existential {
   init()
   func doIt() -> Bool