Revert of Remove register index/code indirection

src/x64/code-stubs-x64.h

Index: src/x64/code-stubs-x64.h
diff --git a/src/x64/code-stubs-x64.h b/src/x64/code-stubs-x64.h
index d3f92c530c19a2d00e12db33142180d69fe59004..1344400d48eff7f70642b8cb621d00dfa3f7cf75 100644
--- a/src/x64/code-stubs-x64.h
+++ b/src/x64/code-stubs-x64.h
@@ -294,15 +294,13 @@
     Register GetRegThatIsNotRcxOr(Register r1,
                                   Register r2,
                                   Register r3) {
-      for (int i = 0; i < Register::kNumRegisters; i++) {
-        Register candidate = Register::from_code(i);
-        if (candidate.IsAllocatable()) {
-          if (candidate.is(rcx)) continue;
-          if (candidate.is(r1)) continue;
-          if (candidate.is(r2)) continue;
-          if (candidate.is(r3)) continue;
-          return candidate;
-        }
+      for (int i = 0; i < Register::NumAllocatableRegisters(); i++) {
+        Register candidate = Register::FromAllocationIndex(i);
+        if (candidate.is(rcx)) continue;
+        if (candidate.is(r1)) continue;
+        if (candidate.is(r2)) continue;
+        if (candidate.is(r3)) continue;
+        return candidate;
       }
       UNREACHABLE();
       return no_reg;