[llvm] [RISCV] Allow spilling to unused Zcmp Stack (PR #125959)

[Sam Elliott via llvm-commits]

lenary wrote:

:/ I will try to get my head around the V-specific parts of framelowering, it could be that we're allocating specific frame indexes for vector registers which end up overlapping with push/pop.

https://github.com/llvm/llvm-project/pull/125959