Clean up NullCheck generation and record stats about it.

This removes redundant code from the generators and allows for easier
stat recording.

Change-Id: Iccd4368f9e9d87a6fecb863dee4e2145c97851c4
diff --git a/compiler/optimizing/optimizing_compiler_stats.h b/compiler/optimizing/optimizing_compiler_stats.h
index ecbe371..3717926 100644
--- a/compiler/optimizing/optimizing_compiler_stats.h
+++ b/compiler/optimizing/optimizing_compiler_stats.h
@@ -62,6 +62,8 @@
   kSelectGenerated,
   kRemovedInstanceOf,
   kInlinedInvokeVirtualOrInterface,
+  kImplicitNullCheckGenerated,
+  kExplicitNullCheckGenerated,
   kLastStat
 };
 
@@ -137,6 +139,8 @@
       case kSelectGenerated : name = "SelectGenerated"; break;
       case kRemovedInstanceOf: name = "RemovedInstanceOf"; break;
       case kInlinedInvokeVirtualOrInterface: name = "InlinedInvokeVirtualOrInterface"; break;
+      case kImplicitNullCheckGenerated: name = "ImplicitNullCheckGenerated"; break;
+      case kExplicitNullCheckGenerated: name = "ExplicitNullCheckGenerated"; break;
 
       case kLastStat:
         LOG(FATAL) << "invalid stat "