stage2: fix handling of error unions as return type

* LLVM backend: fix phi instruction not respecting `isByRef`
   - Also fix `is_non_null` not respecting `isByRef`
 * Type: implement abiSize for error unions
07691db3aeAndrew Kelley committed on 10/17/2021, 10:36:12 PMparent6534f2e
4 files changedLine totals unavailable