Re-reland: 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 09289cf3122417ef598af8d3ef2d22761b5984be..4ba5892b541e8fb00966ded40fdb87ce5c4fcfdd 100644
--- a/src/x64/code-stubs-x64.h
+++ b/src/x64/code-stubs-x64.h
@@ -294,13 +294,15 @@ class RecordWriteStub: public PlatformCodeStub {
     Register GetRegThatIsNotRcxOr(Register r1,
                                   Register r2,
                                   Register r3) {
-      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;
+      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;
+        }
       }
       UNREACHABLE();
       return no_reg;