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/graph_visualizer.cc b/compiler/optimizing/graph_visualizer.cc
index f7fd910..12c6988 100644
--- a/compiler/optimizing/graph_visualizer.cc
+++ b/compiler/optimizing/graph_visualizer.cc
@@ -153,7 +153,7 @@
     }
 
     const uint8_t* base = disassembler_->GetDisassemblerOptions()->base_address_;
-    if (instruction_set_ == kThumb2) {
+    if (instruction_set_ == InstructionSet::kThumb2) {
       // ARM and Thumb-2 use the same disassembler. The bottom bit of the
       // address is used to distinguish between the two.
       base += 1;