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/optimizing/scheduler_test.cc b/compiler/optimizing/scheduler_test.cc
index dfc1633..75dce81 100644
--- a/compiler/optimizing/scheduler_test.cc
+++ b/compiler/optimizing/scheduler_test.cc
@@ -43,22 +43,22 @@
   ::std::vector<CodegenTargetConfig> test_config_candidates = {
 #ifdef ART_ENABLE_CODEGEN_arm
     // TODO: Should't this be `kThumb2` instead of `kArm` here?
-    CodegenTargetConfig(kArm, create_codegen_arm_vixl32),
+    CodegenTargetConfig(InstructionSet::kArm, create_codegen_arm_vixl32),
 #endif
 #ifdef ART_ENABLE_CODEGEN_arm64
-    CodegenTargetConfig(kArm64, create_codegen_arm64),
+    CodegenTargetConfig(InstructionSet::kArm64, create_codegen_arm64),
 #endif
 #ifdef ART_ENABLE_CODEGEN_x86
-    CodegenTargetConfig(kX86, create_codegen_x86),
+    CodegenTargetConfig(InstructionSet::kX86, create_codegen_x86),
 #endif
 #ifdef ART_ENABLE_CODEGEN_x86_64
-    CodegenTargetConfig(kX86_64, create_codegen_x86_64),
+    CodegenTargetConfig(InstructionSet::kX86_64, create_codegen_x86_64),
 #endif
 #ifdef ART_ENABLE_CODEGEN_mips
-    CodegenTargetConfig(kMips, create_codegen_mips),
+    CodegenTargetConfig(InstructionSet::kMips, create_codegen_mips),
 #endif
 #ifdef ART_ENABLE_CODEGEN_mips64
-    CodegenTargetConfig(kMips64, create_codegen_mips64)
+    CodegenTargetConfig(InstructionSet::kMips64, create_codegen_mips64)
 #endif
   };