Remove different OpNames that target the same ID

[khyperia]

There's a bit of unfortunate interaction in the way we link binaries. Here's an order of operations:

1. We emit OpNames for everything, to make it easier for us to look at binaries and debug them.
2. We link the module, merging identical types etc. into the same construct. Relevant OpNames get rewritten to point at the same construct.
3. We strip dead code, types, etc.

This becomes a problem: things like ZSTs and other extremely common types are emitted a lot, with many different names. They're all merged in step 2, but then we have an absolute heap of names all pointing at the same ID, and that ID happens to be used in a few places, so we barf gazillions of strings into the binary.

So, only keep the first name we encounter for any particular ID. This will likely be the "incorrect" name (probably a name that's actually never used and would have been DCE stripped if we never merged identical constructs), but oh well, that name does correspond to the type, close enough.

This reduces ark's spir-v binary size from 743KiB to 93KiB

[eddyb]

Should we be deduplicating so aggressively? Wouldn't it make more sense to only deduplicate types that connect imports and exports (and therefore must match)?

[khyperia]

yeah, probably! that gets way more complicated, though, especially because various things in SPIR-V allow duplication, and various other things don't (e.g. you can only have one OpTypeInt of a certain size, but you can have multiple OpTypeStructs that are the same). It's a whole mess that isn't super worth it (especially since even the basic system we have now has known bugs in it).