[Wasm GC] Automatically make RefCast heap types more precise (#5704)

We already did this for nullablilty, and so for the same reasons we should do it
for heap types as well. Also, I realized that doing so would solve #5703, which
is the new test added for TypeRefining here.

The fuzz bug solved here is that our analysis of struct gets/sets will skip
copy operations - a read from a field that is written into it. And we skip
fallthrough values while doing so, since it doesn't matter if the read goes
through an if arm or a cast. An if would automatically get a more precise
type during refinalize, so this PR does the same for a cast basically.

Fixes #5703

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