Minimal support for several SPIR-V features (either new, or rare edge cases, some Rust-GPU specific).

[eddyb]

This PR contains support for these SPIR-V oddities:

• ID-taking annotations (e.g. OpDecorateId, OpExecutionModeId)
  • obsoletes Carry float flags through lowering and lifting #48
• type operands (e.g. FPFastMathDefault, OpUntypedPtrAccessChainKHR)
  • in theory, this could get rid of TypeOrConst, but it also feels a bit hacky
• (illegal) cycles between global variables
  • only Rust-GPU might be able to produce this

EDIT: moved a couple of commits to this other PR (which had been accidentally closed months ago):

• spv: minimal OpSpecConstantOp and OpConstantFunctionPointerINTEL support.  #22

(opened as draft because I want to make sure there's nothing else missing for 0.5.0)