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/compiled_method.cc b/compiler/compiled_method.cc
index 111469f..fc6a717 100644
--- a/compiler/compiled_method.cc
+++ b/compiler/compiled_method.cc
@@ -61,14 +61,14 @@
 
 size_t CompiledCode::CodeDelta(InstructionSet instruction_set) {
   switch (instruction_set) {
-    case kArm:
-    case kArm64:
-    case kMips:
-    case kMips64:
-    case kX86:
-    case kX86_64:
+    case InstructionSet::kArm:
+    case InstructionSet::kArm64:
+    case InstructionSet::kMips:
+    case InstructionSet::kMips64:
+    case InstructionSet::kX86:
+    case InstructionSet::kX86_64:
       return 0;
-    case kThumb2: {
+    case InstructionSet::kThumb2: {
       // +1 to set the low-order bit so a BLX will switch to Thumb mode
       return 1;
     }
@@ -80,14 +80,14 @@
 
 const void* CompiledCode::CodePointer(const void* code_pointer, InstructionSet instruction_set) {
   switch (instruction_set) {
-    case kArm:
-    case kArm64:
-    case kMips:
-    case kMips64:
-    case kX86:
-    case kX86_64:
+    case InstructionSet::kArm:
+    case InstructionSet::kArm64:
+    case InstructionSet::kMips:
+    case InstructionSet::kMips64:
+    case InstructionSet::kX86:
+    case InstructionSet::kX86_64:
       return code_pointer;
-    case kThumb2: {
+    case InstructionSet::kThumb2: {
       uintptr_t address = reinterpret_cast<uintptr_t>(code_pointer);
       // Set the low-order bit so a BLX will switch to Thumb mode
       address |= 0x1;