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

[Francis Visoiu Mistrih via llvm-commits]

https://github.com/francisvm commented:

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

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