[llvm] [RISCV] Add preserve_none calling convention support (PR #223920)

[via llvm-commits]

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vincent-163 wrote:

I'm doing a binary translation project called COGBT that uses LLVM as the translation backend to run x86 programs on RISCV. Registers are passed as function parameters and musttail calls are used to jump between translation units. PreserveNone helps maximize number of guest registers passed via host registers instead of via memory CPUEnv struct.

https://github.com/llvm/llvm-project/pull/223920