[Wasm GC] Fix GlobalStructInference reasoning on unoptimizability (#5381)

We have a data structure there, typeGlobals, which maps types to the list of
globals for that type. Previously we used the convention of not having an
entry in the map to mean that a type is unoptimizable. However, this was not
used consistently, and in fact one place could insert to the map in a dangerous
way: a subtype's global is added to the list of globals of the super, and
typeGlobals[super].add(sub-global) would then effectively make an unoptimizable
super into an optimizable one.

To fix that, check for unoptimizability before propagating sub-globals. We do
still use the convention of not keeping data in typeGlobals for unoptimizable
things as it is a minor optimization to avoid wasted work.

Fixes #5379 

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