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

[Djordje Todorovic via llvm-commits]

djtodoro wrote:

> > > We already have one pass that does this optimisation: RISCVLoadStoreOptimizer (in PreSched2). Your "PostAlloc" pass should be merged into this pass, as they are doing the same thing, at the same point in the pass pipeline (I will also add @djtodoro as a reviewer as he worked on that pass). The Pre Allocation pass can be left separate.
> > 
> > 
> > They aren't really doing the same thing are they? This pass uncombines the pseudo instructions that didn't register allocate correctly. The existing pass combines small loads into wider loads.
> 
> They aren't, but I think it would be good to have only one early pairing-related pass and one late pairing-related pass, especially if we're going to run the late passes at such a similar time. I realise this PR isn't the one that's going to make the existing late pairing pass support Zilsd.

I agree with this.

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