When reifying a shape transition chain, look for the nearest previous
shape in the transition chain that has a property table already, and
use that as the starting point.
This achieves two things:
1. We do less work when reifying property tables that already have
partial property tables earlier in the chain.
2. This enables adding properties to a shape without performing a
transition. This will be useful for initializing runtime objects
with way fewer allocations. See next patch. :^)
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