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/code_generator_arm_vixl.cc b/compiler/optimizing/code_generator_arm_vixl.cc
index 90f3ae8..edd3072 100644
--- a/compiler/optimizing/code_generator_arm_vixl.cc
+++ b/compiler/optimizing/code_generator_arm_vixl.cc
@@ -2568,7 +2568,7 @@
   if (!skip_overflow_check) {
     UseScratchRegisterScope temps(GetVIXLAssembler());
     vixl32::Register temp = temps.Acquire();
-    __ Sub(temp, sp, Operand::From(GetStackOverflowReservedBytes(kArm)));
+    __ Sub(temp, sp, Operand::From(GetStackOverflowReservedBytes(InstructionSet::kArm)));
     // The load must immediately precede RecordPcInfo.
     ExactAssemblyScope aas(GetVIXLAssembler(),
                            vixl32::kMaxInstructionSizeInBytes,
@@ -5303,7 +5303,7 @@
   vixl32::Label less, greater, done;
   vixl32::Label* final_label = codegen_->GetFinalLabel(compare, &done);
   DataType::Type type = compare->InputAt(0)->GetType();
-  vixl32::Condition less_cond = vixl32::Condition(kNone);
+  vixl32::Condition less_cond = vixl32::Condition::None();
   switch (type) {
     case DataType::Type::kBool:
     case DataType::Type::kUint8: