[effects] Record reads and writes of the GC heap (#3657)

Just as reads and writes to memory can interfere with each other, reads and
writes of GC objects can interfere with each other. This PR adds new `readsHeap`
and `writesHeap` fields to EffectAnalyzer to account for this interference. Note
that memory accesses can never alias with GC heap accesses, so they are
considered separately. Similarly, it would be possible to prove that different
GC heap accesses never interfere with each other based on the accessed types,
but that's left to future work.

Fixes #3655.

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