Fix a bug in ParallelMoveResolver::AllocateScratchRegister, where we end up allocating PC on ARM.

tests/language/memory_swap_test.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test that the ParalleMoveResolver in the VM uses valid registers
+// when requiring scratch registers.
+
+main() {
+  // Iterate enough to ensure optimizing `spillingMethod`.
+  for (int i = 0; i < 100000; i++) {
+    spillingMethod(i, () => 0);
+  }
+}
+
+spillingMethod(what, obfuscate) {
+  // Define lots of variables to get a few in stack.
+  var a = obfuscate();
+  var b = obfuscate();
+  var c = obfuscate();
+  var d = obfuscate();
+  var e = obfuscate();
+  var f = obfuscate();
+  var g = obfuscate();
+  var h = obfuscate();
+  var i = obfuscate();
+  var j = obfuscate();
+  var k = obfuscate();
+  var l = obfuscate();
+  var m = obfuscate();
+  var n = obfuscate();
+  var o = obfuscate();
+  var p = obfuscate();
+  var q = obfuscate();
+  var r = obfuscate();
+  var s = obfuscate();
+  var t = obfuscate();
+  var u = obfuscate();
+  var v = obfuscate();
+
+  // Swap all variables, in the hope of a memory <-> memory swap operation.
+  while (what == 42) {
+    a = b;
+    b = a;
+    c = d;
+    d = c;
+    e = f;
+    f = e;
+    g = h;
+    h = g;
+    i = j;
+    j = i;
+    k = l;
+    l = k;
+    m = n;
+    n = m;
+    o = p;
+    p = o;
+    q = r;
+    r = q;
+    s = t;
+    t = s;
+    u = v;
+    v = u;
+    what++;
+  }
+
+  // Keep all variables alive.
+  return a + b + c + d + e + f + g + h + i + j + k + l + m + n + o
+       + p + q + r + s + t + u + v;
+}