Ports constant propagation to Rust. The algorithm is broadly similar to the TS
version, and most of the differences come from the slightly different HIR data
model (operands are instruction indices not identifier ids). What this means is
that the Constants map that we build up is really only used for variables that
existed in the original program, and only comes into play with instructions like
LoadLocal and StoreLocal. Other instructions such as Binary just look up their
operands directly, ie they load the referenced instruction to check if both
left/right are primitives.
Note that with SSA form and the index-based operands we could actually get rid
of StoreLocal/LoadLocal completely, which would further simplify constant
propagation. However:
* we'd need to add a Phi instruction kind, not a big deal but it diverges even
more
* more importantly, it makes it super hard to implement LeaveSSA
That second point is a deal-breaker so unless someone has a great idea for how
to exit SSA form without having Load/Stores, let's keep them.
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