[llvm] [RISCV] Add zero-store merging (PR #192662)

[Djordje Todorovic via llvm-commits]

djtodoro wrote:

> > hi @topperc, I am wondering if there is any other comment?
> 
> It looks like some of your test cases are already handle by SelectionDAG. The way the PR is structured, I can't tell if this patch changes anything from the current codegen. We like to see new test cases added in their own commit so it shows the current codegen. Then a commit that adds the optimization and updates the test to show the new codegen.

I split the change into pre-commit test, and MIR Pass + test changes, but yeah, latest `SelectionDAG` does handle most of `zero-store` merging, so I will close this PR.

https://github.com/llvm/llvm-project/pull/192662