Previously we only autogen the availability for ops that are
direct instantiating `SPV_Op` and expected other subclasses of
`SPV_Op` to define aggregated availability for all ops. This is
quite error prone and we can miss capabilities for certain ops.
Also it's arguable to have multiple levels of subclasses and try
to deduplicate too much: having the availability directly in the
op can be quite explicit and clear. A few extra lines of
declarative code is fine.
Reviewed By: mravishankar
Differential Revision: https://reviews.llvm.org/D95236
167fb9b4b4