[llvm] [RISCV] Lower i64 load/stores to ld/sd with Zilsd. (PR #139808)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

For reference, on Arm (both 32-bit and 64-bit), we use a merged load/store for volatile operations, but non-volatile operations are optimized in post-isel passes.  It's not necessarily bad to merge early, but it's harder to tell if merging is profitable.

https://github.com/llvm/llvm-project/pull/139808