Added vectorization stats.

Rationale:
Provides a (somewhat crude) quantative way to detect changes in
loop vectorization and idiom recognition (e.g. by means of market
scans, or just inspecting the same application before/after a change).

Test: market scan
Change-Id: Ic85938ba2b33c967de3159742c60301454a152a0
diff --git a/compiler/optimizing/loop_optimization.cc b/compiler/optimizing/loop_optimization.cc
index f8f4eb2..6b2697b 100644
--- a/compiler/optimizing/loop_optimization.cc
+++ b/compiler/optimizing/loop_optimization.cc
@@ -625,6 +625,7 @@
       TryAssignLastValue(node->loop_info, main_phi, preheader, /*collect_loop_uses*/ true)) {
     Vectorize(node, body, exit, trip_count);
     graph_->SetHasSIMD(true);  // flag SIMD usage
+    MaybeRecordStat(stats_, MethodCompilationStat::kLoopVectorized);
     return true;
   }
   return false;
@@ -1724,6 +1725,7 @@
                 vector_length_,
                 is_unsigned,
                 is_rounded));
+            MaybeRecordStat(stats_, MethodCompilationStat::kLoopVectorizedIdiom);
           } else {
             GenerateVecOp(instruction, vector_map_->Get(r), vector_map_->Get(s), type);
           }