Atj-pre-translation-remove-dead-if-branches
Pre-translation step performed by ATJ:
removal of dead if branches.
This is done in both the deep and shallow embedding approach.
If the test of an if is t,
we replace the if with the `then' branch;
if the test of an if is nil,
we replace the if with the `else' branch.
Note that the previous translation step
may turn mbts in if tests into ts,
so it is appropriate for this pre-translation step
to come after the previous one.
We use the remove-dead-if-branches system utility.
No other code is needed to do this in ATJ.
