SLP/RISCV: add negative test for llrint, increase coverage (#69940)

To follow-up on a06be8a2 (SLP/RISCV: add negative test for lrint), add a
negative test for llvm.llrint as well, and increase the coverage to
cover vectors of length 2, 4, and 8, and the i32 variant of lrint, in
preparation to get SLPVectorizer to vectorize both lrint and llrint.
This is now possible with the recent change 98c90a13 (ISel: introduce
vector ISD::LRINT, ISD::LLRINT; custom RISCV lowering).