* Add some assertions to make sure instructions are not none. I tested
   all these with master branch as well and made sure the behavior tests
   still passed with the assertions intact (along with a handful of
   callsite updates).
 * Fix Sema.resolveMaybeUndefValAllowVariablesMaybeRuntime not noticing
   that interned values are comptime-known. This was causing all kinds
   of chaos.
 * Fix print_air writeType calling tag() without checking for ip_index