[llvm] [RISCV] Remove custom isel lowering of i64 to Zilsd load/store. (PR #169067)

[Sam Elliott via llvm-commits]

lenary wrote:

@christian-herber-nxp what was the reason for doing this at O0 rather than e.g. Os or Oz?

https://github.com/llvm/llvm-project/pull/169067