[spirv] Add script to auto-generate SPIR-V op template from spec

[spirv] Add script to auto-generate SPIR-V op template from spec

SPIR-V has a JSON grammar file that defines the syntax of SPIR-V
instructions. However, its lacks fine-grained constraints on
instruction operands; those information is only available as
natural language sentences in the SPIR-V spec, which also contains
the detailed documentation for each SPIR-V instruction.

This CL pulls information from both the JSON grammar and HTML
spec. It right now uses the former to deduce the arguments and
results (with coarse-grained constraints) and the latter for
documentation. In the future we can add the functionality to
match certain natural language sentences for more fine-grained
constraints, but right now the developer is expected to update
the generated op definition. This should serve as a nice
bootstrap step to save efforts.

PiperOrigin-RevId: 257821205