[Wasm GC] Track struct.new and struct.set separately in ConstantFieldPropagation (#4064)

Previously we tracked them in the same way. That means that we did the same
when seeing if either a struct.new or a struct.set can write to the memory that
is read by a struct.get, where the rule is that if either type is a subtype of the
other then they might. But with struct.new we know the precise type, which
means we can do better.

Specifically, if we see a new of type B, then only a get of a supertype of B can
possibly read that data: it is not possible for our struct of type B to appear in
a location that requires a subtype of B. Conceptually:

A = type struct
B = type extends A
C = type extends B

x = struct.new<B>

struct.get<A>(y) // x might appear here, as it can be assigned to a
                 // variable y of a supertype

struct.get<C>(y) // x cannot appear here

This allows more devirtualization. It is a followup for #4052 that implements
a TODO from there.

The diff without whitespace is simpler.

0 files changed, 0 insertions, 0 deletions