Keep resolved String in HLoadString.

For the following reasons:
- Avoids needing to do a lookup again in CodeGenerator::EmitJitRoots.
- Fixes races where we the string was GC'ed before CodeGenerator::EmitJitRoots.
- Makes it possible to do GVN on the same string but defined in different
  dex files.

Test: test-art-host, test-art-target
Change-Id: If2b5d3079f7555427b1b96ab04546b3373fcf921
diff --git a/compiler/optimizing/optimizing_compiler.cc b/compiler/optimizing/optimizing_compiler.cc
index 4bf5b08..297500b 100644
--- a/compiler/optimizing/optimizing_compiler.cc
+++ b/compiler/optimizing/optimizing_compiler.cc
@@ -1205,7 +1205,7 @@
   }
   MaybeRecordStat(MethodCompilationStat::kCompiled);
   codegen->BuildStackMaps(MemoryRegion(stack_map_data, stack_map_size), *code_item);
-  codegen->EmitJitRoots(code_allocator.GetData(), roots, roots_data, dex_cache);
+  codegen->EmitJitRoots(code_allocator.GetData(), roots, roots_data);
 
   const void* code = code_cache->CommitCode(
       self,