Rename HNativeDebugInfo to HNop

We can generalize HNativeDebugInfo to be used as a Nop (i.e. no
instructions are generated), and give it the option of having an
environment to keep the current HNativeDebugInfo logic working.

Test: art/test/testrunner/testrunner.py --host --64 --optimizing -b
Change-Id: I06b3a36e8b124bcda858d2c9cd8ff0ab21caea36
diff --git a/compiler/optimizing/scheduler.cc b/compiler/optimizing/scheduler.cc
index 8f18ccf..d228aba 100644
--- a/compiler/optimizing/scheduler.cc
+++ b/compiler/optimizing/scheduler.cc
@@ -718,7 +718,7 @@
   //    HLoadException
   //    HMemoryBarrier
   //    HMonitorOperation
-  //    HNativeDebugInfo
+  //    HNop
   //    HThrow
   //    HTryBoundary
   // TODO: Some of the instructions above may be safe to schedule (maybe as