* Module.addBreak and addBreakVoid return zir.Inst.Index not Ref
   because Index is the simpler type and we never need a Ref for these.
 * astgen: make noreturn stuff return the unreachable_value and avoid
   unnecessary calls to rvalue()
 * breakExpr: avoid unnecessary access into the tokens array
 * breakExpr: fix incorrect `@intCast` (previously this unsafely
   casted an Index to a Ref)