Refcounting added a second way to represent "no attrs" -- NULL as a hcls.
This was causing guards to fail since a check would be written for one
but then it would fail when objects came in with the other.

So canonicalize to using NULL as the "no attributes" hcls.

It can still be helpful to turn them off, but now that the refcounter non-determinism
has been fixed I think it's time to have these back on.

this let's minibenchmark/raytrace.py run with enabled interpreter + OSR (but disabled bjit)

(we had already copied their int freelist)

I guess it was originally turned on for the extra debugging it provides,
but it also a decent amount faster than system malloc/free
(though I think we could do better still).

Not sure how I missed the dict.cpp file

Put a "expected: reffail" on any test that the tester thought was failing
(which includes a few that currently fail due to the llvm-only mode)

Used a different word (vs "fail") so that we can see whether it was failing
before refcounting or not.

A couple build issues left though.

since we need to look at this at every deinitFrame.  calculateNumUserVisibleVRegs()
was showing up in the profile.

This might be less important under pgo (which is currently broken), but seems
like a good thing to try anyway.

Conflicts:
	src/codegen/irgen/irgenerator.cpp

Conflicts:
	src/codegen/ast_interpreter.cpp
	src/codegen/baseline_jit.cpp
	src/codegen/baseline_jit.h

Conflicts:
	src/runtime/objmodel.cpp

And add some extra ref checking.  The rewriter didn't verify that it didn't
over-decref which made this hard to find.