This is hopefully the first patch in the series of patches adding some
missing SPIR-V ops to MLIR over the next weeks/months, starting with
something simple: `OpEmitVertex` and `OpEndPrimitive`. Since the ops
have no input and outputs, and the only condition is "This instruction
must only be used when only one stream is present.", which I don't think
can be validate at the instruction level in isolation, I set
`hasVerifier` to 0. I hope I didn't miss anything, but I'm more than
happy to address any comments.
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