Replace "zombie" system with post-legalization SPIR-T validation.

[eddyb]

"Zombies" are a form of deferred errors (mostly to ignore errors caused in unused code), for both:

• concepts representable in SPIR-V, but not legal (e.g. OpenCL-only pointer operations)
• concepts not even representable (often implemented as "zombie"-annotated OpUndef values)
  • this is often not a true spec limitation, only an implementation one, e.g. an Intel OpenCL extension could be used for function pointers, and OpSpecConstantOp can express non-trivial constant pointers (in both cases, these would only serve as inputs to legalization, which is expected to rewrite them away - see below)

However, a primary goal of Rust-GPU is to legalize the Rust dynamic semantics into legal SPIR-V, and we're currently inconsistent, where some aspects are not checked before legalization (e.g. variables containing pointers) so that legalization has a chance to rewrite them away - but others use "zombies" and can only compile if "actually unused".

In the past, I've considered making further compromises to keep this system around (perhaps for a bit of nostalgia?), but if I try to be objective about this, Rust-GPU has a better chance at supporting more Rust code if the validation is deferred, and our post-legalization error reporting nowadays is arguably lossless (as e.g. inlining generates custom debuginfo now, for both this usecase, and panic "backtraces").

So I think what we can do is replicate the same checks we have for zombies, but as something more akin to "spirv-val for SPIR-T" (with only the few rules we care about for now)

• see also: https://github.com/EmbarkStudios/spirt/issues/2

(I did start on something like this a month or two ago, but it was slowed down and overcomplicated by trying to do it piecemeal, and it should be quicker if zombies are all going away)