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/instruction_simplifier.cc b/compiler/optimizing/instruction_simplifier.cc
index 189d5ae..2bd2d5f 100644
--- a/compiler/optimizing/instruction_simplifier.cc
+++ b/compiler/optimizing/instruction_simplifier.cc
@@ -250,7 +250,7 @@
   DataType::Type type = mul->GetPackedType();
   InstructionSet isa = codegen_->GetInstructionSet();
   switch (isa) {
-    case kArm64:
+    case InstructionSet::kArm64:
       if (!(type == DataType::Type::kUint8 ||
             type == DataType::Type::kInt8 ||
             type == DataType::Type::kUint16 ||
@@ -259,8 +259,8 @@
         return false;
       }
       break;
-    case kMips:
-    case kMips64:
+    case InstructionSet::kMips:
+    case InstructionSet::kMips64:
       if (!(type == DataType::Type::kUint8 ||
             type == DataType::Type::kInt8 ||
             type == DataType::Type::kUint16 ||