[fast-isel] Add a preemptive fix for the case where we fail to materialize an
immediate in a register.  I don't believe this should ever fail, but I see no
harm in trying to make this code bullet proof.

I've added an assert to ensure my assumtion is correct.  If the assertion fires
something is wrong and we should fix it, rather then just silently fall back to
SelectionDAG isel.