[Mem2Reg] Complete new enum phi lifetimes.
When multi-block `alloc_stack`s of enum type with users are promoted,
lifetimes of stored values are completed to compensate for the legality
of not `destroy_addr`ing the `alloc_stack` in blocks where no
non-trivial value is known to be stored (the none case block of a
`switch_enum` of a load_borrow of the `alloc_stack`, e.g.).
Here, phis created during promotion are lifetime completed as well.