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

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

I just did a close reading, and I think I'm happy enough? The below are broadly just nits. I don't think any of them are required to land this.

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