[llvm] [RISCV] Allow prefetch hints without Zicbop. (PR #191027)

[Sam Elliott via llvm-commits]

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lenary wrote:

Oh, we could probably do with a test of `ori` with x0 that is disassembled as `prefetch`, just to show we know this will happen.

https://github.com/llvm/llvm-project/pull/191027