optimizing: Refactor statistics to use OptimizingCompilerStats helper

Remove all copies of 'MaybeRecordStat', replacing them with a single
OptimizingCompilerStats::MaybeRecordStat helper.

Change-Id: I83b96b41439dccece3eee2e159b18c95336ea933
diff --git a/compiler/optimizing/dead_code_elimination.cc b/compiler/optimizing/dead_code_elimination.cc
index c31c66a..787296d 100644
--- a/compiler/optimizing/dead_code_elimination.cc
+++ b/compiler/optimizing/dead_code_elimination.cc
@@ -359,7 +359,7 @@
       DCHECK(!inst->IsControlFlow());
       if (inst->IsDeadAndRemovable()) {
         block->RemoveInstruction(inst);
-        MaybeRecordStat(MethodCompilationStat::kRemovedDeadInstruction);
+        MaybeRecordStat(stats_, MethodCompilationStat::kRemovedDeadInstruction);
       }
     }
   }