[llvm] ISel: introduce vector ISD::LRINT, ISD::LLRINT; custom RISCV lowering (PR #66924)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

Please can you move the tests to a stacked pre-commit so we can see the codegen diff?

https://github.com/llvm/llvm-project/pull/66924