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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

I've realized the limitations of this patch:

- rv32 can only do i32, and rv64 can only do i64.
- I don't know if the ISD::LLRINT lowering on rv32 is correct (it's identical to the rv64 output, but I haven't added a RUN line for it).

Should I add asserts to guard against these?

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