[RegisterConfiguration] Streamline access to arch defaults, simplify Registers.

test/cctest/test-code-stubs-arm64.cc

Index: test/cctest/test-code-stubs-arm64.cc
diff --git a/test/cctest/test-code-stubs-arm64.cc b/test/cctest/test-code-stubs-arm64.cc
index f5a40789beb2492399425c1cc8fa596136d86a2c..2edb9236752b89e52ef9eb3f03d08236a3c2ddc1 100644
--- a/test/cctest/test-code-stubs-arm64.cc
+++ b/test/cctest/test-code-stubs-arm64.cc
@@ -74,8 +74,9 @@ ConvertDToIFunc MakeConvertDToIFuncTrampoline(Isolate* isolate,
   int source_reg_offset = kDoubleSize;
   int reg_num = 0;
   for (; reg_num < Register::kNumRegisters; ++reg_num) {
-    Register reg = Register::from_code(reg_num);
-    if (reg.IsAllocatable()) {
+    if (RegisterConfiguration::Crankshaft()->IsAllocatableGeneralCode(
+            reg_num)) {
+      Register reg = Register::from_code(reg_num);
       if (!reg.is(destination_reg)) {
         queue.Queue(reg);
         source_reg_offset += kPointerSize;
@@ -103,8 +104,9 @@ ConvertDToIFunc MakeConvertDToIFuncTrampoline(Isolate* isolate,
 
   // // Make sure no registers have been unexpectedly clobbered
   for (--reg_num; reg_num >= 0; --reg_num) {
-    Register reg = Register::from_code(reg_num);
-    if (reg.IsAllocatable()) {
+    if (RegisterConfiguration::Crankshaft()->IsAllocatableGeneralCode(
+            reg_num)) {
+      Register reg = Register::from_code(reg_num);
       if (!reg.is(destination_reg)) {
         __ Pop(ip0);
         __ cmp(reg, ip0);