Fix liveness for remove dead of parfors (and other IR extensions)

[ehsantn]

No description provided.

[sklam]

The error from test_alias_parfor_extension without the fix in ir_utils.py looks very much like the kind of errors I am seeing in my SSA attempt. And it would make sense because SSA will create many aliases as new versions of a variable is created.

[ehsantn]

Makes sense. One potentially related issue might be copy propagation not fully working since SSA generates extra assignments.

[sklam]

The test is failing on windows and 32-bit linux. It needs a skip test.

[ehsantn]

done.

[stuartarchibald]

Thanks for the patch. I've checked this manually and I'm of the view that the fix makes sense.

@sklam I recommend merge unless you have any other thoughts related to SSA?

[sklam]

This is good for merging. I've checked this against my SSA branch. It fixes less tests then I have hoped.