Assert with `--enable-gc-nn-locals`, take 2

[askeksa-google]

Re-open of https://github.com/WebAssembly/binaryen/issues/4554

The assert is still hit after https://github.com/WebAssembly/binaryen/pull/4555.

[kripken]

Ah, sorry, there were two issues here, and the reducer gave me a testcase earlier that found just one... I should have checked on the original testcase at the end. Fix in https://github.com/WebAssembly/binaryen/pull/4561 and I verified the original testcase is actually resolved.