VectorUtils: mark lrint, llrint as trivially vectorizable (#69945)

With the recent change 98c90a13 (ISel: introduce vector ISD::LRINT,
ISD::LLRINT; custom RISCV lowering), it is now possible for
SLPVectorizer, LoopVectorize, and Scalarizer to operate on llvm.lrint
and llvm.llrint, with vector codegen for the RISC-V target. Make a
trivial change to VectorUtils, and update the corresponding tests.