[llvm] [RISCV] RISCV vector calling convention (2/2) (PR #79096)

[via llvm-commits]

bgra8 wrote:

@4vtomat the [LLVM dev policy](https://llvm.org/docs/DeveloperPolicy.html#patch-reversion-policy) mandates reverts instead of fixes forward to keep the tree healthy.

Please revert first to unblock all other teams that are hindered by this issue, investigate the issue offline, and then reapply the fixed patch.



https://github.com/llvm/llvm-project/pull/79096