Non-nullable locals mode not producing non-nullable locals

[askeksa-google]

I have been trying out the --enable-gc-nn-locals flag on some output modules from dart2wasm. I was surprised to find that the optimized Binaryen output when using non-nullable locals was bigger than the one using only nullable locals, even though for the original modules, the one using non-nullable locals was smaller.

I looked at the output and noticed a few things:

• The optimized non-nullable version doesn't actually contain any non-nullable locals. This is the case even for a non-optimizing wasm-to-wat pass-through.
• The code contains many superfluous ref.as_non_null, i.e. on a non-nullable value or as input to something accepting a nullable value.

It looks like the code was processed with non-nullable locals in mind, but for the output, the locals somehow became nullable and all the ref.as_non_null were inserted to make the types match up.

Since at least some of these could potentially trap, it would seem that removing them is only valid with the --ignore-implicit-traps option. Thus, a fix for #3934 could be valuable for doing a fair comparison between having non-nullable locals and not.