A64: Use a scope utility to allocate scratch registers.

src/a64/assembler-a64.h

Index: src/a64/assembler-a64.h
diff --git a/src/a64/assembler-a64.h b/src/a64/assembler-a64.h
index a2c93df2ae274bf540e1c41f25c9b09e9d70f6f4..c50d5c20025e5dda79db36e614bd5e61380ad2c5 100644
--- a/src/a64/assembler-a64.h
+++ b/src/a64/assembler-a64.h
@@ -461,6 +461,11 @@ class CPURegList {
     return list_;
   }
 
+  inline void set_list(RegList new_list) {
+    ASSERT(IsValid());
+    list_ = new_list;
+  }
+
   // Combine another CPURegList into this one. Registers that already exist in
   // this list are left unchanged. The type and size of the registers in the
   // 'other' list must match those in this list.