Use down_cast instead of reinterpret_cast in Optimizing codegens.

Change-Id: Ifa23023ffaca631a4f6b5745dd7492c39521a26f
diff --git a/compiler/optimizing/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index 78d5851..ca9a154 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -46,7 +46,7 @@
 
 static constexpr int kC2ConditionMask = 0x400;
 
-#define __ reinterpret_cast<X86_64Assembler*>(codegen->GetAssembler())->
+#define __ down_cast<X86_64Assembler*>(codegen->GetAssembler())->
 
 class NullCheckSlowPathX86_64 : public SlowPathCodeX86_64 {
  public:
@@ -343,7 +343,7 @@
 };
 
 #undef __
-#define __ reinterpret_cast<X86_64Assembler*>(GetAssembler())->
+#define __ down_cast<X86_64Assembler*>(GetAssembler())->
 
 inline Condition X86_64Condition(IfCondition cond) {
   switch (cond) {