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

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

Index: test/cctest/test-code-stubs-mips.cc
diff --git a/test/cctest/test-code-stubs-mips.cc b/test/cctest/test-code-stubs-mips.cc
index ce577da46bdf3b7a5119882e0fa254d48b1e6007..149a1742f8aad45b8ffcf9d3d755c8ef1fe0fccb 100644
--- a/test/cctest/test-code-stubs-mips.cc
+++ b/test/cctest/test-code-stubs-mips.cc
@@ -82,8 +82,9 @@ ConvertDToIFunc MakeConvertDToIFuncTrampoline(Isolate* isolate,
   int source_reg_offset = kDoubleSize;
   int reg_num = 2;
   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)) {
         __ push(reg);
         source_reg_offset += kPointerSize;
@@ -111,8 +112,9 @@ ConvertDToIFunc MakeConvertDToIFuncTrampoline(Isolate* isolate,
 
   // Make sure no registers have been unexpectedly clobbered
   for (--reg_num; reg_num >= 2; --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)) {
         __ lw(at, MemOperand(sp, 0));
         __ Assert(eq, kRegisterWasClobbered, reg, Operand(at));