[llvm] RISCV: Implement isLoadFromStackSlot/isStoreToStackSlot for rvv (PR #120524)

[Philip Reames via llvm-commits]

preames wrote:

> @preames want to adopt this if you're touching RISCV regalloc?

Possibly.  Let's chat about it when we talk Wednesday.  I'm missing a bit of context on motivation, and mechanism.  

https://github.com/llvm/llvm-project/pull/120524