ART: Make InstructionSet an enum class and add kLast.

Adding InstructionSet::kLast shall make it easier to encode
the InstructionSet in fewer bits using BitField<>. However,
introducing `kLast` into the `art` namespace is not a good
idea, so we change the InstructionSet to an enum class.
This also uncovered a case of InstructionSet::kNone being
erroneously used instead of vixl32::Condition::None(), so
it's good to remove `kNone` from the `art` namespace.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I6fa6168dfba4ed6da86d021a69c80224f09997a6
diff --git a/simulator/code_simulator_arm64.h b/simulator/code_simulator_arm64.h
index 0542593..8b66529 100644
--- a/simulator/code_simulator_arm64.h
+++ b/simulator/code_simulator_arm64.h
@@ -25,6 +25,7 @@
 #include "aarch64/simulator-aarch64.h"
 #pragma GCC diagnostic pop
 
+#include "arch/instruction_set.h"
 #include "code_simulator.h"
 
 namespace art {
@@ -48,7 +49,7 @@
   vixl::aarch64::Simulator* simulator_;
 
   // TODO: Enable CodeSimulatorArm64 for more host ISAs once Simulator supports them.
-  static constexpr bool kCanSimulate = (kRuntimeISA == kX86_64);
+  static constexpr bool kCanSimulate = (kRuntimeISA == InstructionSet::kX86_64);
 
   DISALLOW_COPY_AND_ASSIGN(CodeSimulatorArm64);
 };