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

[Sam Elliott via llvm-commits]

lenary wrote:

The change with this PR is that `PseudoLD_RV32_OPT` are now introduced in two places, in ISel and in the Zilsd Optimisation Pass. They are still removed in the load/store optimizer.

I think with this change, we might generate a `PseudoLD_RV32_OPT` which we want to push a `%lo` into (using MergeBaseOffset) - but we have to be very careful because we need to know we can add 4 to it without overflow in case we need to split it into two loads. The same effectively applies for stores.

I think we already have code for this for Zdinx, so hopefully we can reuse that. I don't think the lack of MergeBaseOffset changes should prevent this change.

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