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

test/unittests/compiler/move-optimizer-unittest.cc

Index: test/unittests/compiler/move-optimizer-unittest.cc
diff --git a/test/unittests/compiler/move-optimizer-unittest.cc b/test/unittests/compiler/move-optimizer-unittest.cc
index 5ccd0c67277ba2d7197f639c66f533a6b6b5382f..4c693846670393cb3e34fb1d8cd2a2f73d65d160 100644
--- a/test/unittests/compiler/move-optimizer-unittest.cc
+++ b/test/unittests/compiler/move-optimizer-unittest.cc
@@ -106,11 +106,9 @@ TEST_F(MoveOptimizerTest, RemovesRedundant) {
 
 TEST_F(MoveOptimizerTest, RemovesRedundantExplicit) {
   int first_reg_index =
-      RegisterConfiguration::ArchDefault(RegisterConfiguration::TURBOFAN)
-          ->GetAllocatableGeneralCode(0);
+      RegisterConfiguration::Turbofan()->GetAllocatableGeneralCode(0);
   int second_reg_index =
-      RegisterConfiguration::ArchDefault(RegisterConfiguration::TURBOFAN)
-          ->GetAllocatableGeneralCode(1);
+      RegisterConfiguration::Turbofan()->GetAllocatableGeneralCode(1);
 
   StartBlock();
   auto first_instr = EmitNop();