Clean up Scoped-/ArenaAlocator array allocations.

Change-Id: Id718f8a4450adf1608306286fa4e6b9194022532
diff --git a/compiler/dex/mir_optimization_test.cc b/compiler/dex/mir_optimization_test.cc
index 199bc27..be05b80 100644
--- a/compiler/dex/mir_optimization_test.cc
+++ b/compiler/dex/mir_optimization_test.cc
@@ -280,7 +280,7 @@
 
   void DoPrepareMIRs(const MIRDef* defs, size_t count) {
     mir_count_ = count;
-    mirs_ = reinterpret_cast<MIR*>(cu_.arena.Alloc(sizeof(MIR) * count, kArenaAllocMIR));
+    mirs_ = cu_.arena.AllocArray<MIR>(count, kArenaAllocMIR);
     uint64_t merged_df_flags = 0u;
     for (size_t i = 0u; i != count; ++i) {
       const MIRDef* def = &defs[i];