Revert of Remove register index/code indirection

src/ia32/code-stubs-ia32.h

Index: src/ia32/code-stubs-ia32.h
diff --git a/src/ia32/code-stubs-ia32.h b/src/ia32/code-stubs-ia32.h
index dabd9c08f3850a66013bd169f605539ca056db20..c09b27b773e2a538fa16498d380bbe1e78f5a75f 100644
--- a/src/ia32/code-stubs-ia32.h
+++ b/src/ia32/code-stubs-ia32.h
@@ -320,15 +320,13 @@
     Register GetRegThatIsNotEcxOr(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(ecx)) 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(ecx)) continue;
+        if (candidate.is(r1)) continue;
+        if (candidate.is(r2)) continue;
+        if (candidate.is(r3)) continue;
+        return candidate;
       }
       UNREACHABLE();
       return no_reg;