Support for CONST_STRING in optimizing compiler.

Change-Id: Iab8517bdadd1d15ffbe570010f093660be7c51aa
diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index 5d504c6..ec9af73 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -539,6 +539,7 @@
   M(Div)                                                   \
   M(FloatConstant)                                         \
   M(LoadClass)                                             \
+  M(LoadString)                                            \
   M(Neg)                                                   \
   M(NewArray)                                              \
   M(ParallelMove)                                          \