Optimizing: Improve const-string code generation.

For strings in the boot image, use either direct pointers
or pc-relative addresses. For other strings, use PC-relative
access to the dex cache arrays for AOT and direct address of
the string's dex cache slot for JIT.

For aosp_flounder-userdebug:
  - 32-bit boot.oat: -692KiB (-0.9%)
  - 64-bit boot.oat: -948KiB (-1.1%)
  - 32-bit dalvik cache total: -900KiB (-0.9%)
  - 64-bit dalvik cache total: -3672KiB (-1.5%)
    (contains more files than the 32-bit dalvik cache)
For aosp_flounder-userdebug forced to compile PIC:
  - 32-bit boot.oat: -380KiB (-0.5%)
  - 64-bit boot.oat: -928KiB (-1.0%)
  - 32-bit dalvik cache total: -468KiB (-0.4%)
  - 64-bit dalvik cache total: -1928KiB (-0.8%)
    (contains more files than the 32-bit dalvik cache)

Bug: 26884697
Change-Id: Iec7266ce67e6fedc107be78fab2e742a8dab2696
diff --git a/compiler/optimizing/graph_visualizer.cc b/compiler/optimizing/graph_visualizer.cc
index 4b5b919..bcdd3e7 100644
--- a/compiler/optimizing/graph_visualizer.cc
+++ b/compiler/optimizing/graph_visualizer.cc
@@ -367,6 +367,10 @@
         << load_class->NeedsAccessCheck() << std::noboolalpha;
   }
 
+  void VisitLoadString(HLoadString* load_string) OVERRIDE {
+    StartAttributeStream("load_kind") << load_string->GetLoadKind();
+  }
+
   void VisitCheckCast(HCheckCast* check_cast) OVERRIDE {
     StartAttributeStream("check_kind") << check_cast->GetTypeCheckKind();
     StartAttributeStream("must_do_null_check") << std::boolalpha