Move dexCacheStrings from ArtMethod to Class

Adds one load for const strings which are not direct.

Saves >= 60KB of memory avg per app.
Image size: -350KB.

Bug: 17643507
Change-Id: I2d1a3253d9de09682be9bc6b420a29513d592cc8

(cherry picked from commit f521f423b66e952f746885dd9f6cf8ef2788955d)
diff --git a/compiler/optimizing/code_generator_arm64.cc b/compiler/optimizing/code_generator_arm64.cc
index 7a8b941..0fc4307 100644
--- a/compiler/optimizing/code_generator_arm64.cc
+++ b/compiler/optimizing/code_generator_arm64.cc
@@ -1553,7 +1553,8 @@
 
   Register out = OutputRegister(load);
   codegen_->LoadCurrentMethod(out);
-  __ Ldr(out, HeapOperand(out, mirror::ArtMethod::DexCacheStringsOffset()));
+  __ Ldr(out, HeapOperand(out, mirror::ArtMethod::DeclaringClassOffset()));
+  __ Ldr(out, HeapOperand(out, mirror::Class::DexCacheStringsOffset()));
   __ Ldr(out, MemOperand(out.X(), CodeGenerator::GetCacheOffset(load->GetStringIndex())));
   __ Cbz(out, slow_path->GetEntryLabel());
   __ Bind(slow_path->GetExitLabel());