[Wasm GC] Fix TypeRefining corner case with uncreated types (#4500)

This pass ignores reads from structs - it only cares about writes (during a
create or a struct.set). That makes sense since we want to refine the type
of fields to more specific things based on what is actually written to them.
However, a corner case was missed: If we ignore reads, the pass may
"cleverly" optimize to something that is no longer valid to read from. How
that happens is if there is no info at all for a type - no sets or news, so all
we have is a read, which as mentioned before we ignore, so we think we
have nothing at all for that type, and can do arbitrary stuff with it. But then
the arbitrary replacement can be invalid to read from, say if it has fewer
fields.

To handle that, just emit an unreachable. If all we have is a get but no
new then there cannot be an instance here at all. (That's only true in a
closed world, of course, but this entire pass assumes that anyhow.)

0 files changed, 0 insertions, 0 deletions