[llvm] [RISCV] Use Zilsd Pseudos in ISel (PR #169580)

[Brandon Wu via llvm-commits]

4vtomat wrote:

I'm not familiar with `MergeBaseOffset` but I think it works on load/store with same offset and `ZilsdLoadStoreOptimizer` works on load/store with different(consecutive) offset so maybe they can work fine?

https://github.com/llvm/llvm-project/pull/169580