[clang] [llvm] [RISCV][P-ext] Support Packed "Q-format" Multiply Parts Accumulate (PR #217918)

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TelGome wrote:

> Please don't force push. It makes review harder.

 Sorry about that. I'll stop force-pushing and add changes as new commits going forward so the diff history and review comments stay intact.

https://github.com/llvm/llvm-project/pull/217918