[clang] [llvm] [RISCV] Support spacemit vsmtvdotii externsions (PR #202533)

[Mark Zhuang via cfe-commits]

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zqb-all wrote:

Since fp16/bfp16 is entirely controlled by a separate CSR, there's no need to encode it in the instruction—this was also dropped from the final spec:
https://github.com/spacemit-com/docs-ai/blob/main/en/architecture/ime_extension.md#612-instruction-variants
Note that while this encoding existed in earlier draft versions, it has been explicitly removed in the final specification.​
This patch should simplify and remove the FP16/BFP16-related logic accordingly.

https://github.com/llvm/llvm-project/pull/202533