[spectec] Missing and wrong side conditions in the middle-end

[zilinc]

I think for MemE the side condition about the sequence being non-empty is wrong (https://github.com/WebAssembly/spec/blob/main/spectec/src/middlend/sideconditions.ml#L63).

Also there should be side conditions for SliceE, UpdE and ExtE.

Opening this ticket just for the record. I think I'll do a PR with the flag controlling side conditions on functions, after we backport the middle-end from the WasmDSL repo.