llvm: fix load of packed struct that was initialized through pointers

[jacobly0]

It's possible that the isAbiInt condition could be relaxed to containerLayout == .Packed, but this seems safer for now.

Blocked by llvm/llvm-project#64222
Closes #14200

[andrewrk]

It looks like that issue is not going to be fixed with LLVM 17. I think we must choose one of these options:

• move A packed struct member of a struct isn't equal to itself. #14200 to milestone 0.13.0
• work around the issue in the LLVM backend

[jacobly0]

For the second one, the options I see are:

• Try reducing the scope to only packed structs, and hope it regresses something slightly less than literally everything targetting wasm.
• Do not apply this bugfix when targetting wasm, wasm would continue to have an obscure packed struct miscompilation, but it would be way better than every + miscompiling.

[andrewrk]

Landed in aaf4618 as part of #17391.