LegalizeIntegerTypes: implement PromoteIntRes for xrint (#71055)

Recently, 98c90a13 (ISel: introduce vector ISD::LRINT, ISD::LLRINT;
custom RISCV lowering) introduced vector variants of llvm.lrint,
llvm.llrint, and bundled several tests along with the code change.
However, it forgot to test lrint and llrint on fixed vectors on RISC-V,
and it turns out that that fixed-vectors-lrint.ll requires
PromoteIntRes_XRINT to be implemented. Implement it, and add tests for
fixed-vector lrint, llrint.