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/compiler/driver/compiled_method_storage_test.cc b/compiler/driver/compiled_method_storage_test.cc
index e1ea630..de481ca 100644
--- a/compiler/driver/compiled_method_storage_test.cc
+++ b/compiler/driver/compiled_method_storage_test.cc
@@ -31,7 +31,7 @@
   CompilerDriver driver(&compiler_options,
                         &verification_results,
                         Compiler::kOptimizing,
-                        /* instruction_set_ */ kNone,
+                        /* instruction_set_ */ InstructionSet::kNone,
                         /* instruction_set_features */ nullptr,
                         /* image_classes */ nullptr,
                         /* compiled_classes */ nullptr,
@@ -91,7 +91,7 @@
         for (auto&& f : cfi_info) {
           for (auto&& p : patches) {
             compiled_methods.push_back(CompiledMethod::SwapAllocCompiledMethod(
-                &driver, kNone, c, 0u, 0u, 0u, s, v, f, p));
+                &driver, InstructionSet::kNone, c, 0u, 0u, 0u, s, v, f, p));
           }
         }
       }