enums should always be parseable; check against versions, capabilities, and extensions during validation instead

[dneto0]

Currently, SPIRV-Tools parsing only makes an enum visible if it's in some version of SPIR-V, or is guarded by a capability or extension.
See
https://github.com/KhronosGroup/SPIRV-Tools/blob/main/source/operand.cpp#L66

  // We consider the current operand as available as long as
  // 1. The target environment satisfies the minimal requirement of the
  //    operand; or
  // 2. There is at least one extension enabling this operand; or
  // 3. There is at least one capability enabling this operand.
  //
  // Note that the second rule assumes the extension enabling this operand
  // is indeed requested in the SPIR-V code; checking that should be
  // validator's work.

See also this proposal to enforce this in SPIRV-Headers.
KhronosGroup/SPIRV-Headers#369

Arguably this is the wrong thing to do. In the case of SPV_KHR_cooperative_matrix, it's been argued that the enums like MatrixASignedComponentsKHR in CooperativeMatrixOperands should not have to be specially guarded because they are only used by an instruction which itself is already guarded by a capability.

The parser doesn't have that context, so it can't check the transitive dependency, but the transitive dependency should be sufficient.
The proposal is that the validator should be the one to check, instead of the parser.

Going with this proposal would dovetail nicely with #5332 which further removes version checks in grammar operations.

[dneto0]

cc: @alan-baker

[dneto0]

This is going well.