[threads] Validate shared-polymorphic instructions

[tlively]

Such as ref.eq, i31.get_{s,u}, and array.len. Also validate that
struct and array operations work on shared structs and arrays.

[kripken]

I see fuzz errors after this landed, involving a file modified here, shared-array.wast. E.g. seed 15293465023328618278. If that doesn't reproduce on your machine @tlively you might see it by running the fuzzer for a short bit. If not then I can send you a full reproducer (I can't paste it here as it needs the binary input.dat file that the the fuzzer takes as input).

[kripken]

Ah, I realized I can just base64-encode it. Here:

https://gist.github.com/kripken/8e2fd08f2f95a7bfb0d5808e1a05a1fa

$ base64 -d input.dat.base64 > input.dat
$ bin/wasm-opt input.dat -ttf -all --disable-memory64 --disable-multivalue --closed-world --initial-fuzz=initial.wat

[tlively]

Yep, sorry about that. See #6706