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/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 1cd9142..46474d2 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -589,7 +589,7 @@
  public:
   explicit Dex2Oat(TimingLogger* timings) :
       compiler_kind_(Compiler::kOptimizing),
-      instruction_set_(kRuntimeISA == kArm ? kThumb2 : kRuntimeISA),
+      instruction_set_(kRuntimeISA == InstructionSet::kArm ? InstructionSet::kThumb2 : kRuntimeISA),
       // Take the default set of instruction features from the build.
       image_file_location_oat_checksum_(0),
       image_file_location_oat_data_begin_(0),
@@ -893,13 +893,13 @@
     // Checks are all explicit until we know the architecture.
     // Set the compilation target's implicit checks options.
     switch (instruction_set_) {
-      case kArm:
-      case kThumb2:
-      case kArm64:
-      case kX86:
-      case kX86_64:
-      case kMips:
-      case kMips64:
+      case InstructionSet::kArm:
+      case InstructionSet::kThumb2:
+      case InstructionSet::kArm64:
+      case InstructionSet::kX86:
+      case InstructionSet::kX86_64:
+      case InstructionSet::kMips:
+      case InstructionSet::kMips64:
         compiler_options_->implicit_null_checks_ = true;
         compiler_options_->implicit_so_checks_ = true;
         break;