[mlir][spirv] Move type checks from dialect class to type hierarchy

[antiagainst]

Summary

Types should be checked with the type hierarchy. This should result in
better responsibility division and API surface.

Depends On D76242

[mravishankar]

This is so much nicer! Thanks

mlir/lib/Dialect/SPIRV/SPIRVLowering.cpp

166

You could just make this a dyn_cast<spirv::SPIRVType>()

[antiagainst]

mlir/lib/Dialect/SPIRV/SPIRVLowering.cpp

166

I think it's different. If this is not a SPIR-V type, then dyn_cast<spirv::SPIRVType>() gives us a Type() instead of llvm::None. The former will let the framework to try another conversion rule but the later will directly fail. (Although here it is the last conversion rule so probably doesn't matter that much.) Anyway, this is completely changed in the next commit (https://reviews.llvm.org/D76244). :)

[antiagainst]

Rebased