[llvm] [RISC-V] Only emit multiples of 16 as immediate for cm.push (PR #84935)

[Nemanja Ivanovic via llvm-commits]

nemanjai wrote:

> `CM_PUSH` LGTM. Do we need to do the same for `CM_POP` in `emitEpilogue`?

Argh! Yes indeed! I don't know why I missed that (other than I am porting this from a downstream fix I did which is on LLVM 17 where the fix is in a separate function that computes the immediate and is called in both places).
In any case, I'll fix it.

https://github.com/llvm/llvm-project/pull/84935