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..72d135841652861bdf21582b0bd0d13fa1a2f659 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.
@@ -471,9 +476,12 @@ class CPURegList {
   // in the 'other' list must match those in this list.
   void Remove(const CPURegList& other);
 
-  // Variants of Combine and Remove which take a single register.
+  // Variants of Combine and Remove which take CPURegisters.
   void Combine(const CPURegister& other);
-  void Remove(const CPURegister& other);
+  void Remove(const CPURegister& other1,
+              const CPURegister& other2 = NoCPUReg,
+              const CPURegister& other3 = NoCPUReg,
+              const CPURegister& other4 = NoCPUReg);
 
   // Variants of Combine and Remove which take a single register by its code;
   // the type and size of the register is inferred from this list.