remove some dead code

This peculiar sequence which checked a condition and then asserted the opposite
of it inside the branch seems just plain wrong. We are probably lucky compilers
did not take advantage of the conflicting condition and assumption and conclude
this was dead code and remove the entire function. The remaining
assertion/assumption should now be usable by the compiler.

As far as I can tell, this was broken since this code was first written in
commit 7d0a8b74.

This was picked up by building with warnings enabled but no optimisation. Clang
complains that the call to state_rule_taken_get inside __builtin_assume is
impure. This should not be true and I think Clang is wrong, though I'm surprised
it cannot see that read_raw() is pure. Naturally this only came in when we
started aggressively packing structure members, so would not have been noticed
previously.