[llvm] [RISCV] Adjust RVV stack alignment by ABI (PR #101002)

[Sam Elliott via llvm-commits]

lenary wrote:

I'm not sure if this is the right solution, but that is mostly due to my unfamiliarity with the V extension(s).

>From a quick reading, the alignment requirement depends on the element width, and we can always use a vsetvli to set the element width to 8 bits for spills/reloads, so I think this should be acceptable? maybe?

I think the `*e` ABIs are unstable enough that this should be acceptable to change, but I'm not sure. I wonder whether we should be marking them as more experimental than they have been marked to date.

https://github.com/llvm/llvm-project/pull/101002