Move code item to the data pointer and remove code_item_offset.

This saves 4 bytes on 32bit and 8 bytes on 64bit on ArtMethod.

Also update nterp to directly fetch the code item from the data pointer.

Test: test.py
Bug: 112676029

Change-Id: Ic01f43c7ccf2cbce1ec517478e81362232d36371
diff --git a/compiler/optimizing/optimizing_compiler.cc b/compiler/optimizing/optimizing_compiler.cc
index 392aadd..3c168ba 100644
--- a/compiler/optimizing/optimizing_compiler.cc
+++ b/compiler/optimizing/optimizing_compiler.cc
@@ -1238,7 +1238,7 @@
 
   const DexFile* dex_file = method->GetDexFile();
   const uint16_t class_def_idx = method->GetClassDefIndex();
-  const dex::CodeItem* code_item = dex_file->GetCodeItem(method->GetCodeItemOffset());
+  const dex::CodeItem* code_item = method->GetCodeItem();
   const uint32_t method_idx = method->GetDexMethodIndex();
   const uint32_t access_flags = method->GetAccessFlags();