nir/opt_loop_unroll: Remove unneeded phis if we make progress
Now that SSA values can be derefs and they have special rules, we have to be a bit more careful about our LCSSA phis. In particular, we need to clean up in case LCSSA ended up creating a phi node for a deref. This avoids validation issues with some CTS tests with the new patch, but its possible this we could also see the same problem with the existing unrolling passes.
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