Re-reland: 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 3aa18f250af05b1c45c8b2c4f4cf2f12d650bd52..12a556f19bd474dd8b9d60fa8bc44f72c3d36321 100644
--- a/src/ia32/code-stubs-ia32.h
+++ b/src/ia32/code-stubs-ia32.h
@@ -320,13 +320,15 @@ class RecordWriteStub: public PlatformCodeStub {
     Register GetRegThatIsNotEcxOr(Register r1,
                                   Register r2,
                                   Register r3) {
-      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;
+      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;
+        }
       }
       UNREACHABLE();
       return no_reg;