[llvm] [RISCV][WIP] Add validation of SPIMM for cm.push/pop. (PR #84989)

[Nemanja Ivanovic via llvm-commits]

nemanjai wrote:

LGTM. Just an idea - perhaps we should have a test or two with non-multiples of 16 in `test/MC/RISCV/rv{64|32}zcmp-invalid.s`

https://github.com/llvm/llvm-project/pull/84989