We have a good comment on `TEE` transformation in `RegStackify`:
https://github.com/llvm/llvm-project/blob/547e3456660000a16fc5c2a2f819f1a2b5d35b5d/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp#L613-L632
And I think it can be helpful to have some more comments on how the
`TEE`s createdin `RegStackify` are converted into `LOCAL_TEE`s.
Variable `OldReg` is changed to `DefReg` to be consistent with
`RegStackify`'s comment.