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

[Sam Elliott via llvm-commits]

lenary wrote:

Before I do another review of this code, please can you once again go through the comments I've left and check that you've taken them into account? Even the ones you have marked resolved?

Going forwards, please can you leave reviewers to mark their own comments as resolved - you've marked some of mine as resolved without actually fixing the underlying issues to my satisfaction. I don't know if this is because you made changes but had problems with git, or because I haven't been clear enough with where I see the problems, or something else.

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