[llvm] [llvm][RISCV] Implement Zilsd load/store pair optimization (PR #158640)

[Sam Elliott via llvm-commits]

lenary wrote:

@4vtomat thanks for having the patience to work through all our comments, I do appreciate that this wasn't as quick/easy as one might have hoped. I'm glad with where we got to though, it's good!

https://github.com/llvm/llvm-project/pull/158640