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

src/arm64/macro-assembler-arm64.cc

Index: src/arm64/macro-assembler-arm64.cc
diff --git a/src/arm64/macro-assembler-arm64.cc b/src/arm64/macro-assembler-arm64.cc
index 86ddefa9ad40ea23fe770d8edbe3ef8ea9df62a6..83b33b7abab30ec76d01c7922403a5b0b1cf8f43 100644
--- a/src/arm64/macro-assembler-arm64.cc
+++ b/src/arm64/macro-assembler-arm64.cc
@@ -4129,16 +4129,14 @@ void MacroAssembler::PushSafepointRegistersAndDoubles() {
   PushSafepointRegisters();
   PushCPURegList(CPURegList(
       CPURegister::kFPRegister, kDRegSizeInBits,
-      RegisterConfiguration::ArchDefault(RegisterConfiguration::CRANKSHAFT)
-          ->allocatable_double_codes_mask()));
+      RegisterConfiguration::Crankshaft()->allocatable_double_codes_mask()));
 }
 
 
 void MacroAssembler::PopSafepointRegistersAndDoubles() {
   PopCPURegList(CPURegList(
       CPURegister::kFPRegister, kDRegSizeInBits,
-      RegisterConfiguration::ArchDefault(RegisterConfiguration::CRANKSHAFT)
-          ->allocatable_double_codes_mask()));
+      RegisterConfiguration::Crankshaft()->allocatable_double_codes_mask()));
   PopSafepointRegisters();
 }