spirv: deduplication pass

[Snektron]

This linker pass deduplicates types and constants which are the same. It works by hashing and comparing the type/constant/annotation trees (of which a node is called an "entity"), and checking if all the members are the same.

To hash a type or compare two types, their trees are recursively walked and each node is hashed or compared. This is not done with anything smart for now, because we need to mind to handle recursive types in a way that yields the same hash for different types that have the same recursive structure. Recursive types are handled by storing all pointers in an ArrayHashMap, and if the pointer is already there (and so this is a recursive type), the index is hashed or compared instead of recursively processing it. For two equal types, these recursive pointers will have the same indices. This might seem like it would yield false positives, but in those cases there would also be a different type structure (for instance, an extra struct node in the tree), which is caught by the regular comparison code.

For some related yap on the linker, see the previous merge request, #19337