[arm] Share constant pool entries in snapshot.

src/assembler.h

Index: src/assembler.h
diff --git a/src/assembler.h b/src/assembler.h
index 65976676b4c0f9d4d291dc65c24996e262a54aae..1c190b03bf8629c6d38a9ac6da4191b3581dfb1a 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -1167,6 +1167,7 @@ class ConstantPoolEntry {
     return merged_index_;
   }
   void set_merged_index(int index) {
+    DCHECK(sharing_ok());
     merged_index_ = index;
     DCHECK(is_merged());
   }